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PHYSIOGRAPHY. 



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PHYSIOGRAPHY: 

AN INTRODUCTION TO THE STUDY 
OF NATURE. 



, BY 

TrH.^HUXLEY, F. R. S. 



WITH ILLUSTRATIONS AND COLOURED PLATES. 



NEW YORK : 

D. APPLETON AND COMPANY, 

I, 3, AND 5 BOND STREET. 

r888. 



Plate V: 



W 




OF Colours 



I 



\ LoNDOfi a 

i aND LQweR 
J LONVONTERnAR 



CLAY 

£R 

tertiaries . 



3 



WEALD Clay 



Geological MAP of the basin of the Thames. 



Lias 



ChilteriL Hills 



Wbrthi DcnoLs 




^ealevel 



Geological Section across the Thames Basin 



IS 88 



Authorized Edition. 



Exchange 
Ub'ry. Amer. Univ. Grad. School 
Mar. 9,1937 






F 



PREFACE. 

Nearly nine years ago, I was invited, by the Managers 
of the London Institution, to take part in a series of courses 
of Educational Lectures ; which were intended to initiate 
young people in the elements of Physical Science. 

My course was to be the first of the series ; and I made 
use of the opportunity, thus afforded me, to put into 
practical shape the ideas, which I had long entertained 
and advocated, respecting the proper method of approach- 
ing the study of Nature. 

It appeared to me to be plainly dictated by common 
sense, that the teacher, who wishes to lead Iiis pupil to 
form a clear mental picture of the order which pervades 
the multiform and endlessly shifting phenomena of nature, 
should commence with the familiar facts of the scholar's 
daily experience ; and that, from the firm ground of such 
experience, he should lead the beginner, step by step, to 
remoter objects and to the less readily comprehensible 
relations of things. In short, that the knowledge of the 
child should, of set purpose, be made to grow, in the same 



VI PREFACE. 

manner as that of the human race has spontaneously 
grown, 

I conceived that a vast amount of knowledge respecting 
natural phenomena and their interdependence, and even 
some practical experience of scientific method, could be 
conveyed, with all the precision of statement, which is what 
distinguishes science from common information ; and, yet, 
without overstepping the comprehension of learners who 
possessed no further share of preliminary educational dis- 
cipline, than that which falls to the lot of the boys and girls 
who pass through an ordinary primary school. And I 
thought, that, if my plan could be properly carried out, it 
would not only yield results of value in themselves, but 
would facilitate the subsequent entrance of the learners into 
the portals of the special sciences. 

I undertook, therefore, to deliver twelve lectures, not on 
any particular branch of natural knowledge, but on natural 
phenomena in general ; and I borrowed the title of " Physio- 
graphy," which had already been long applied, in a different 
sense, to a department of mineralogy, for my subject; 
inasmuch as I wished to draw a clear line of demarcation, 
both as to matter and method, between it and what is 
commonly understood by " Physical Geography." 

Many highly valuable compendia of Physical Geography, 
for the use of scientific students of that subject, are extant ; 
but, in my judgment, most of the elementary works I have 
seen, begin at the wrong end, and too often terminate in an 
omnium gatherzim of scraps of all sorts of undigested and 
unconnected information ; thereby entirely destroying the 
educational value of that study which Kant justly termed 
the "propaedeutic of natural knowledge." 



PREFACE. vii 

I do not think that a description of the earth, which 
commences, by telling a child that it is an oblate spheroid, 
moving round the sun in an elliptical orbit: and ends, 
without giving him tlie slightest hint towards under- 
standing the ordnance map of his own county ; or any 
suggestion as to the meaning of the phenomena offered by 
the brook which runs through his village, or the gravel pit 
whence the roads are mended ; is calculated either to interest 
or to instruct. And the attempt to convey scientific con- 
ceptions, without the appeal to observation, which can alone 
give such conceptions firmness and reality, appears to me 
to be in direct antagonism to the fundamental principles 
of scientific education. 

" Physiography " has very little to do with this sort of 
" Physical Geography." My heaiers were not troubled with 
much about latitudes and longitudes ; the heights of moun- 
tains, depths of seas ; or the geographical distribution of 
kangaroos and Compositts. Neglecting such pieces of 
information — of the importance of which, in their proper 
places, I entertain no doubt — I endeavoured to give them, 
in very broad, but, I hope, accurate, outlines, a view of the 
"place in nature" of a particular district of England, the 
basin of the Thames ; and, to leave upon their minds the 
impression, that the muddy waters of our metropolitan river ; 
the hills between which it flows ; the breezes which blow over 
it ; are not isolated phenomena, to be taken as understood 
because they are familiar. On the contrary, I endeavoured 
to show that the application of the plainest and simplest 
processes of reasoning to any one of these phenomena, 
suffices to show, lying behind it, a cause, which again suggests 
another ; until, step by step, the conviction dawns upon the 
learner that, to^ altam to even an elementary conception 



viii PREFACE. 

of what goes on in his parish, he must know something 
about the universe ; that the pebble he kicks aside would 
not be what it is and where it is, unless a particular chapter 
of the earth's history, finished untold ages ago, had been 
exactly what it was. 

It was necessary to illustrate my method by a concrete 
case ; and, as a Londoner addressing Londoners, I selected 
the Thames, and its basin, for my text. But any intelligent 
teacher will have no difficulty in making use of the river 
and river basin of the district, in which his own school is 
situated, for the same purpose. 

The lectures on Physiography were delivered at the 
London Institution in 1869 ; and I repeated them, at 
South Kensington, in 1870. Verbatim reports were taken 
on the former occasion, as it was my intention to publish 
the course. But I am sorry to say that, in this, as in other 
cases, I have found a great gulf fixed between intention 
to publish and its realization. 

Seeing a book through the press is a laborious and time- 
wasting affair ; greatly aggravated, in cases such as the 
present, by the necessity of superintending the execution of 
maps and figures. And, as I never could muster up the 
courage, or find the time, to undertake the business, the 
manuscript remained untouched until last year. 

I then had the good fortune to be able to obtain the 
services of my friend Mr. Rudler, with whose extensive 
knowledge of various branches of physical science, I was 
well acquainted ; and, in whose conscientious accuracy, 
as an editor, I knew I could place implicit confidence. 

In preparing the substance of the lectures for the press. 



PREFACE. ix 

Mr. Rudler has entirely fulfilled my expectations, and has 
made many valuable suggestions and additions. I have 
entirely re-written such parts of the work as I thought I 
could improve ; I have added to others ; and I have 
carefully revised the proofs of every ciiapter. 

I trust therefore that the book maybe useful to both learners 
and teachers ; but, I am most concerned, that the latter 
should find in it the ground-work of an introduction to the 
study of nature, on which their practical experience will 
enable them to erect a far better superstructure than that 
which I have been able to raise. 

T. H. H. 

London, 

Is! cnj ember, 1 877. 



PREFACE TO THE THIRD EDITION. 

The large demand for this work for educational purposes, 
during the last three years, has suggested the issue of 
a new and cheaper edition, which has been carefully 
revised. 

I beg leave to offer my thanks to various correspondents 
who have called my attention to errors of the pre:-s, or 
ambiguities of expression in the former editions. 



T. H. H. 



London, 
June, iSSo. 



CONTENTS. 



CHAPTER I. 

PAGE 

THE THAMES . . . . „ I 



CHAPTER n. 
SPRINGS .......... 21 

CHAPTER in. 
RAIN AND DEW 39 

CHAPTER IV. 

THE CRYSTALLISATION OF WATER: SNOW AND ICE . . ^S 

CHAPTER V. 

EVAPORATION . . » , . . . . 66 

CHAPTER VI. 

THE ATMOSPHERE . . 75 



xii CONTENTS. 

CHAPTER VII. 

KAGH 

THE CHEMICAL COMPOSITION OF PURE WATER . . . lOO 
CHAPTER VIII. 

THE CHEMICAL COMPOSITION OF NATURAL WATERS. . IIS 

CHAPTER IX. 

THE WORK OF RAIN AND RIVERS , I30 

CHAPTER X. 

ICE AND ITS WORK -. . . . 150 

CHAPTER XI. 

THE SEA AND ITS WORK 166 

CHAPTER XII. 

EARTHQUAKES AND VOLCANOES 185 

CHAPTER XIII. 

SLOW MOVEMENTS OF THE LAND 205 

CHAPTER XIV. - , 

LIVING MATTER AND THE EFFECTS OF ITS ACTIVITY ON 
THE DISTRIBUTION OF TERRESTRIAL SOLIDS, FLUIDS, 
AND GASES — DEPOSITS FORMED BY THE REMAINS 
OF PLANTS . 217 



CONTENTS. ziii 

CHAPTER XV. 

FAGB 

THE FORMATION OF LAND BY ANIMAL AGENCIES — 

CORAL LAND 246 



CHAPTER XVI. 

THE FORMATION OF LAND BY ANIMAL AGENCIES — 

FORAMINIFERAL LAND 260 



CHAPTER XVH. 

THE GEOLOGICAL STRUCTURE OF THE BASIN OF THE 
THAMES ; AND THE INTERPRETATION OF THAT 
STRUCTURE 272 



CHAPTER XVni. 

THE DISTRIBUTION OF LAND AND WATER 299 

CHAPTER XIX. 

THE FIGURE OF THE EARTH — CONSTRUCTION OF MAPS. 3^7 

CHAPTER XX. 

THE MOVEMENTS OF THE EARTH 337 

CHAPTER XXI. 
THE SUN ....-, 359 



LIST OF ILLUSTRATIONS. 



FIG. PAGE 

1. How to find the North point 8 

2. Magnetic Declination . lo 

3. Hill-shading by means of hachures 12 

4. Contour-lines round a lull 14 

5. Section across the Thames basin, showing the form of the 

ground 16 

6. Formation of a spring 23 

7. Section of Hampstead Heath 26 

8. Horizontal permeable and impermeable strata 29 

9. Inclined permeable and impermeable strata 29 

10. Effect of a fault on the position of a spring or ^^■ell ... 30 

11. Diagrammatic Section across the London basin 31 

12. Rain-gauge 49 

13. Daniell's hygrometer 52 

14. Ice in water 57 

15. Rock crystal . . - 58 

16. Snow-crystals 61 

17. Ice-flowers 63 

18. Hair hygrometer 70 

19. Dry and wet bulb thermometers . , 71 

20. Decomposition of red oxide of mercury . . , . , 77 

21. Torricelli's experiment . go 

22. Times weather chart 93 

23. Z>«//y iVirzf/5 ^barometer chart ... 96 



xvi LIST OF ILLUSTRATIONS. 

FIG. FAGB 

24. Daily Telegraph barometer chart 97 

25. Decomposition of water by electricity loi 

26. Oxygen and hydrogen from decomposition of water . . . 103 

27. Decomposition of water by means of sodium 107 

28. Decomposition of water by means of heated iron .... 108 

29. Formation of water by combustion of dry hydrogen . . . 112 

30. Bridge of travertine at Clermont, Auvergne 12.1 

31. Stalactites and stalagmites, Isle of Cakly 123 

32. The Grand CaKon, Colorado 137 

33. Self-established drainage system 138 

34. River-valley worn through gravel and LonJon clay . , . . 139 

35. Delta of the Nile 144 

36. Catchment-basin and delta of a river 145 

37. Delta of the Mississippi 146 

38. Hope's experiment on the contraction of water 152 

39. Glacier of Zermatt 156 

40. Motion of a glacier 157 

41. Lateral and medial moraines 160 

42. Roches Moutonnees Creek, Colorado 162 

43. The Needles, Isle of Wight 169 

44. Map of the Atlantic, showing course of the Gulf Stream . . 174 

45. Section of the Atlantic Ocean between Sandy Hook (near 

New York) and Bermuda, a distance of 700 nautical miles. 175 

46. Currents in water by heat 177 

47. Currents in water by cold .177 

48. Chart of the estuary of tlie Thames between the Nore and 

Margate 182 

49. Plain of marine denudation 183 

50. Diagrammatic Section of a Volcano 190 

51. Diagi'ammatic Section of a Cinder Cone ' . 191 

52. Breached Volcanic Cones, Auvergne ....'.,... 194 

53. Diagrammatic Section of a Volcano, with Dykes and Minor 

Cones ... 194 

54. Summit of Vesuvius in 1756 . 195 

55. Summit of Vesuvius in 1767 196 

56. Vesuvius and Monte Somma . . . . 198 



LIST OF ILLUSTRATIONS. xvii 

FIG. TACn 

57. Graham Island, 1831 19^ 

58. Beehive Geyser, Yellowstone Park, Colorado 201 

59. Marble Column from Temple of Jupiter Serapis .... 207 

60. Strata deposited in a basin 214 

61. Strata thrown into a basin-shape 214 

62. Section from Abingdon to the Isle of Wight 215 

63. Fossil fruit {Nipadites ellipticus) from the London clay, 

Sheppey 230 

64. Microscopic section of diatomaceous deposit, Mourne Moun- 

tains, Ireland 231 

65. Section of coal measures . . 235 

66. Sdgniaria ficoixies ; a coal measure fossil 236 

67. Sigillaria attached to stigmarian roots 237 

68. Cuboidal block of coal split along natural planes .... 238 

69. Microscopic section of Better Bed coal, Yorkshire . . . 239 

70. Caryophyllia Stnithii, a coral-polype from the coast of Devon- 

shire 250 

71. Diagrammatic section of a single cup-coral, to show the 

general structure of the polype, and the relation of the 

skeleton to the soft parts 251 

72. Thecopsammia socialis, Pourtales 252 

73. Diagrammatic section of an island surrounded by a fringing- 

reef 258 

74. Diagrammatic section of an island surrounded by a barrier- 

reef, with intervening lagoon 258 

75. Diagrammatic section of a coral island, or atoll 258 

76. 77. Deep-sea sounding apparatus used by the Challenger . . 262 

78. Section of the Atlantic sea-bed between Newfoundland and 

Ireland 264 

79. Globigerina bulloides, D'Orb. ; Orbulina universa, D'Orb. ; 

a coccosphere ; a coccolith, two views • . . . . . 265 

80. Section exposed in Cannon Street, London, 185 1 .... 275 

81. Map of the Valley of the Thames, between Kingston and 

Woolwich 279 

82. Section across the Valley of the Wandle, sho"iving high-lsv°l 

gravels and valley-gravel .... „ 280 

b 



xviii LIST OF ILLUSTRATIONS. 

FIG. PAGR 

83. Cyrena {^Corbicula) fluminalis 281 

84. The Mammoth {Elephas primigenius) 283 

85. A palaeolithic implement, from Gray's Imi Lane .... 285 
Z6. A neolithic implement from the Thames at London . . . 285 

87. Nautilus centralis, from the London clay 289 

88. Microscopic section of chalk from Sussex 292 

89. Atlantic ooze from a depth of 2,250 fathoms .... 292 

90. Map showing the efTect of an upheaval of the sea-bottom 

around the British Isles, to the extent of lOO fathoms . . 302 

91. Map of the World, showing direction of the principal chains 

of mountains 304 

92. Diagrammatic section across Eurasia 306 

93. Diagrammatic section across North America 312 

94. Diagrammatic section across South America 312 

95. Map of the Arctic regions 314 

96. Continental or land hemisphere 315 

97. Oceanic or water hemisphere 3^5 

98. Disappearance of a ship at sea 318 

99. Ship approaching shore. Curvature of the sea . . . . 319 
100. Enlargement of the horizon by ascending a hill . . . . 321 
xoi. The earth within the star-sphere , 322 

102. Difference between polar and equatorial diameters of the 

earth 326 

103. Coordinates of a point ... 327 

104. Parallels of latitude 328 

105. Lines of longitude 328 

io6. Orthographic projection . . 332 

107. Globular projection 333 

108. Conical development 334 

109. Mercator's projection 335 . 

1 10. Polar projection 336 

111. Diagram to illustrate the effect of changing the position of the 

earth's axis in relation to the sun 339 

112. Diagram showing the earth's relation to the sun at different 

seasons . , . 3C0 

113. The earth at the summer solstice . ■ ... . 35^ 



LIST OF ILLUSTRATIONS. xix 

(■ lO. PAOB 

114. The earth at an equinox .... , 353 

115. Diagram to illustrate perihelion and aphelion 354 

116. Zones of the earth's surface 357 

117. Figure to show that apparent magnitude depends upon the 

visual angle 360 

118. Comparative sizes of the sun and the earth 363 

119. The great sun-spot of 1865 as it appeared on Oct. 15 . . 364 

1 20. Apparent paths of sun spots at different times of the year . 366 

121. The corona and solar prominences as seen during a total 

eclipse in 1851 367 

122. The formation of the solar spectrcm . . . ... 369 



COLOURED PLATES. 

PLATE 

1. Contoured Map of the Thames Basin Frontispiece. 

II. Map of Great Britain and Ireland divided into 

River-basins facing page 2o 

III. Typical forms of Clouds „ 42 

IV. Hyetographical or Rain Map ot England and 

Wales „ 48 

W, Geological Map of the Basin of the Tiiames . . ,, 288 



PHYSIOGRAPHY. 

CHAPTER I. 

THE THAMES. 

No spot in the world is better known than London, and no 
spot in London is better known than London Bridge. Let 
the reader suppose that he is standing upon this bridge, and, 
needless of the passing stream of traffic, looks down upon 
the river as it runs below. It matters little on which side of 
the bridge lie may chance to stand ; whether he look up 
the river or down the river, above bridge or below bridge. 
In either case he will find himself in the presence of a 
noble stream measuring, when broadest, nearly a sixth of 
a mile from bank to bank. The quantity of water under 
London Bridge varies considerably, however, at different 
seasons, and even at different hours on the same day. When 
the water is highest, the greatest depth is about thirty 
feet and the width 800 feet ; when the water is lowest, the 
greatest depth is something like tweLe feet and the width 
only 650 feet. This variation in the volume of water shows 
that the river is not at rest, and that its surface is, in fact, 
alternately rising and falling. Moreover, apart from the local 
agitation due to traffic, apart too from the surface ripples 

^ R 



2 PHYSIOGRAPHY. [chap. 

raised by the passing breeze, the whole body of water is 
in a constant state of motion. At one time in the day the 
water sweeps down below bridge in the direction of Green- 
wich and far onwards to the Nore ; after this movement 
has been continued for some hours, it gradually slackens, 
and the water comes almost to a stand-still; then the motion 
begins afresh, but its direction is reversed, the water flowing 
this time towards Westminster and far away up the river ; 
but, after a while, this motion slowly subsides, and is followed 
once more by renewed movement in a contrary direction. 

Every one knows that this regular backward-and-forward 
movement of the great mass of water is due to tidal 
action. For about seven hours, during ebb-tide, the water of 
the Thames runs down towards the sea ; and for about five 
hours, during flood-tide, the movement takes place in th& 
opposite direction, the water being then driven up the 
river. At the end of the ebb-tide the river is shallowest , 
at the end of the flood tide it is deepest. The water at 
London Bridge is consequently twice in every four-and 
twenty hours at its highest and twice at its lowest level. 

As we go up the river, we find the efi'ect of the tide 
gradually diminishing, until at length it ceases to be felt. 
In point of fact, the tidal movement has no influence 
beyond Teddington Weir, some nineteen miles above 
London Bridge. The very name Teddington is indeed said 
to be a corruption of " Tide-end-ton," the town where 
the tide ends. Up to Teddington, then, the Thames is 
a tidal river, the water rising, and falling, and moving, 
fiist in one direction and then contrariwise, at definite 
intervals. The flood tide occupies about two hours in 
coming up from the Nore to London, a distance of about 
40 miles ; that is to say, it is high water at London Bridge 
two iiours later than at the Nore. Then it takes two hours 



I.] THE THAMES. . 3 

more for the gradual rise of the \^ater to its highest level, 
to travel onwards from London to Teddington, though the 
distance between these two points is less than twenty miles. 
The rise of the level of the surface of the Thames, which 
thus passes with great rapidity from the Nore towards 
Teddington, is accompanied by a much slower general 
motion of the water in the same direction, which con- 
stitutes the upward current of the tide ; while a downward 
current in the opposite direction attends the fall of the 
water during the ebb. 

Floating bodies are therefore carried towards Teddington, 
during the flood, and towards the Nore, during the ebb. 
It is almost needless to add that this tidal action is of 
vast service to the port of London, since barges, lighters, 
and other boats are thus enabled at certain periods of the 
day to float up or down the river with little or no 
expenditure of power on the part of the boatmen. 

Above Teddington Weir the motion of the river is 
totally different from that which is observed at London 
Bridge. There is no alternate backward-and-forward motion, 
no regular rise and fall of the water, but the river flows 
onwards in one constant direction, always running down 
towards London. Careful observations at Teddington have 
shown that, with the water at ordinary summer-level, about 
380 million gallons ' flow over the weir every four-and-twenty 
hours. This vast volume of water is swept down past 
London, and ultimately carried out to sea. As the ebb- 
tide runs for about seven hours, whilst the flood lasts only 
five, it is clear that much more water runs down than flows 
up ; and it is in this way that the vast volume of water 
sent down from above Teddington drains away seawards. 

1 Many of the statistical data relating to the Thames have been 
obligingly furnished by Mr. Leach, of the Thames Conservancy Board. 

B 2 



4 PHYSIOGRAPHY. [chap. 

In seeking the origin of the water thus brought down 
by the Thames, it is necessary to trace the river to what 
is commonly called its " source." On ascending, it is ob- 
served that the river grows smaller, the volume of water 
becoming less and less. Thus, at Teddington, the Thames 
is only 250 feet wide at high water, whilst its width at 
London Bridge is about 800 feet. Follov/ing the many 
windings of the river past Windsor, Reading, and Oxford, 
we observe the stream still growing more narrow and 
more shallow, until at Lechlade, in Gloucestershire, 146 
miles from London, the Thames ceases to be navigable. 
At Lechlade the quantity of water running down the river 
has been roughly estimated at something like a hundred 
million gallons per day, or only about one-fourth the 
quantity flowing over Teddington Weir. The main stream 
splits up at Lechlade into a number of smaller streams, 
forming the " head-waters " of the river, and it is by no 
means easy to say which of these streams should be 
followed up in seeking the true source of tlie Thames. 
Nor does it much matter, for the origin of any one of 
them is much the same as the origin of any of the others. 
It is usual, however, to single out one of these streams, 
which takes its rise in a spring near Cirencester, about 170 
miles from London Bridge, and is dignified with the name 
of Thames Head. 

Although the spring at Thames Head is thus popularly 
called the "source" of the river, it should be remem- 
bered that the quantity of water delivered by this 
spring is quite insignificant when compared with that 
derived from the numerous streams which flow into the 
Thames at various points along its course. Every 
tributary helps to swell the bulk of the river by dis- 
charging its water into the main stream ; yet it does not 



I] THE THAMES. • 5 

follow that the river is necessarily increased in width by 
the influx of this water, for it often happens that the 
additional supply is carried off by increased rapidity of 
flow. As the Thames rolls along, it receives a number 
of these feeders, or affluents,^ which empty themselves 
into the river, some on one side and some on the other. 

It is obviously convenient to have some ready means of 
distinguishing the two banks of a river. For this purpose, 
geographers have agreed to call that bank which lies upon 
your right side as you go down towards the sea the right 
bank, and to call the opposite side the left bank. All that 
you have to do then, in order to distinguish the two sides, 
is to stand so that your face is in the direction of the 
mouth of the river, and your back consequently towards 
its source, when the right bank will be upon your right 
hand and the left bank upon your left hand. At Graves- 
end, for example, the right bank is that which forms the 
Kentish shore, while the left bank is on the Essex side. With 
reference therefore to the rivers tributary to the Thames, it is 
said that the Churn, the Colne, the Leach, the Windrush, 
the Evenlode, the Cherwell, the Thame, the Coin, the 
Brent, and the Lea empty themselves into the Thames 
on the left bank; and the Rey, the Cole, the Ock, the 
Kennet, the Loddon, the Wey, the Mole, and the Darent, 
open into the river on its right bank. The relative 
positions of these affluents^ and their relation to the 
Thames, may be seen in the map given in Plate I. 

If a person in a balloon passed at a great height over any 

'^ Affluent, from the Latin ad zxiAfuo, " to flow." The junction of an 
affluent with the main stream is termed the confiuence, or place where 
they "flow together." Thus, the town of Coblenz takes its name 
from the Latin form Confine ntes, in allusion to its position at the 
junction of the Moselle and the Rhine. 



6 PHYSIOGRAPHY. [chap. 

part of the earth's surface, and sketched in outUne what he 
saw directly below, his sketch on a flat surface like this 
page would be called a ma/>. When the portion of country 
thus delineated is but small, the sketch is generally termed 
d. plati ; and if the area depicted consist chiefly of water 
instead of land, it is called a chart Hence we commonly 
speak of the plan of an estate, the map of a country, the 
chart of an ocean. A map of the Thames, then, is simply 
an outline-sketch of the river and neighbouring portion of 
the earth's surface, as would be seen from a balloon pass- 
ing at a great height directly over the country. It is the 
common practice to draw maps in such a position that the 
north is towards the top, and the south towards the bottom ; 
while the east lies on the right hand of the person who 
looks at the map, and the west lies on his left hand. By 
simply looking then at the map, forming Plate I., it is 
seen at once that the Thames, though taking — like most 
rivers — an irregular course, winding first in one direction 
and then in another, nevertheless has, on the whole, a west- 
and-east course ; it flows, in short, from the west towards 
the east. At the same time it is seen that the left shore of 
the river is its northern bank, and the right shore its southern 
bank. It is clear, too, that the tributaries on the left or 
north side flow generally from north to south, whilst those 
on the riglit or south side run generally from south to 
north. 

These terms — north and south, east and west — are terms 
which have a meaning quite independent of local circum- 
stances, and indicate definite directions which can be 
determined in any part of the world and at all times. 
Wlien, in the early part of this chapter, we used the local 
expressions "up the river" and "down the river," "above 
bridge " and " below bridge." it was assumed that the reader 



I.] THE THAMES. ■ 7 

was familiar with the Thames. But to a perfect stranger, 
one who had never seen the river and knew nothing of 
London Bridge, such a method of description would be 
unintelligible. By employing, however, the terms north 
and south, east and west, we are using expressions that 
are familiar to all educated people, since they refer to 
standards of direction universally recognised. It is de- 
sirable to explain how these cardinal points may be 
be determined. 

Of the four points, the south is perhaps the most easily 
found, at least on a sunshiny day. Every morning the sun 
appears to rise slowly in the sky, and mounts to its greatest 
height at noon. At the instant of reaching its greatest 
height, or in other words at exact noon, the sun is pre- 
cisely in the south. If, then, you place yourself in such 
a position as to have the sun shining full in your face 
at that particular time, you must be facing south ; and 
you will consequently have your back to the north, your 
right hand towards the west, and your left towards the 
east. 

As true noon does not always coincide with 12 o'clock, 
as indicated by an ordinary timekeeper, it is necessary to 
explain how it may be determined. Thrust a stick verti- 
cally into the ground, and observe, at different hours of the 
day, the length and direction of its shadow cast by the sun. 
When the sun is rising in the sky, the shadow is thrown 
towards the west ; and when the sun is going down, it is 
thrown towards the east; at noon, however, it inclines 
neither to the east nor to the west, but falls exactly in a 
north- and -south line ; and, moreover, the shadow is then 
shorter than at any other time. If, therefore, you observe 
when the shadow is shortest, that time will be exactly noon. 
The line indicated by the shadow at noon is known as the 



PHYSIOGRAPHY. 



[chap. 



meridian line or mid-day line. That end of the shadow-line 
which is towards the sun points to the south, and the oppo- 
site end to the north. If then a Hne be drawn anywhere 
at right angles across the shadow, the right-hand end of the 
cross-line, as you look to the south, points towards the 
west, and the left hand towards the east. 



POLE STAR 



if ^^ ^a. 

GREAT SEAR 



Fig. I — How to find the North point. 



It is not easy, however, by merely looking at the shadow, 
to say when it is exactly reduced to its shortest length. It 
is well, therefore, to observe the shadow at some time in 
the forenoon and mark its length — say by sticking a pe"- 
in the ground, and then, in the afternoon, to observe the 



I.] THE THAMES. ' 9 

shadow again, when it has reached exactly the same length. 
The afternoon shadow will then be just as much on one 
side of the meridian line as the forenoon shadow was on the 
other side. The mid-day line, or line which runs due north 
and south, will therefore be exactly half-way between the 
two shadows. 

But it is by no means necessary to have daylight in order 
to discover the direction of the cardinal points. If you 
look up into the heavens on a clear starry night, you will 
have no difficulty, in this part of the world, in finding that 
curious group of seven bright stars known as Chaj'Ies's 
Wain, forming part of the Great Bear (Fig. i). A line drawn 
through two of these stars {/?, a) will, if prolonged to about 
five times its length, pass very close to the i^xaoMs Pole-star} 
On a clear night, all the groups of stars appear to move 
slowly round a certain motionless point in the sky, which is 
the north pole of the heavens. That point of the earth 
which is directly opposite the celestial north pole is the 
north pole of the earth. If the explorers in the Alert 
and Discovery could have reached the north pole, they 
would have found the pole-star almost directly overhead. 

^ It is the practice of astronomers to distinguish the several stars of 
a particular group, or constellation, by means of Greek lettei^s. Thus 
the two stars in the Great Bear, known as the Pointers, since a line 
joining them points towards the pole-star, are distinguished in Fig. i by 
the letters a and ;8. The first of these stars would be technically 
described as a Ui'scs Alajoris, or the alpha star of the Great Bear. 
This constellation contains several stars of which only seven conspicuous 
ones are represented in the figure. The pole-star, known also as 
Polaris, is the brightest of a group called the Little Bear, and is con- 
sequently described as a Urscz Minoris. The word "pole," which, 
in common English, signifies a long straight stick, is used in astronomy 
and geography in the sense of the Greek -KiXos {polos), a pivot on which 
everything turns. The heavens appear to turn round the north pole 
as if it were a pivot. 



PHYSIOGRAPHY. 



[chap 



It must be remembered, however, that the pole-star is not 
exactly at the north pole of the heavens, although very near 
to it. By observing the position of the pole-star, the north 
can be determined on a clear night as readily as the south 
may be determined by the sun at noon. 

If, however, the sun is beclouded, so that the heavenly 
bodies are not visible, there is yet another easy means of 
finding the direction of the cardinal points. Let a thin bar 
of steel, or even a needle, be nicely balanced upon a pivot, 
or suspended by a thread, or floated upon a cork in water, 



TRUE NORTH 




Fig. 2. — Magnetic Declination. 



SO that it can turn freely in all directions horizontally; it 
will be found that the bar may be brought to rest in any 
desired position. If, however, the bar be rubbed with a 
magnet, a peculiar change is wrought in the steel, and it 
then no longer exhibits this indifference to direction, but, 
when left free, always takes up a definite position, one end 
pointing in a northerly and the other in a southerly direc- 
tion. This property is taken advantage of in constructing 
the mariner's compass. About two hundred and twenty 



r.] THE THAMES. ' n 

years ago the compass pointed exactly north and south in 
London; but from the year 1660, or thereabouts, the end 
which tends northwards, and is therefore commonly called 
the north pole of the needle, began to turn a little to the 
west; this variation from true north continued until the 
year 181 8, when it reached its greatest divergence, and 
since that time it has been steadily creeping back. The 
divergence of the position of the magnetic needle from 
the true north-and-south line is called its declination, or, 
by nautical men, its variation. In 1818 the declination 
amounted to nearly 25°, and in the present year (1877) it 
is 19° 3' in London; that is to say, the end or pole of 
the needle which turns in a northerly direction, instead of 
pointing due north, points 19° 3' to the west of true 
north. This declination is shown in Fig. 2. Knowing 
the amount of declination, it is easy to make the proper 
allowance and thus find the true points of the compass. 
By means of the compass the direction of the river in all 
its windings may be traced, and the course of the mean- 
dering stream laid down upon a map, as has been done, 
for example, in Plate I. 

This map, however, does something more than show 
simply the direction of the Thames and its tributaries ; it 
gives us, in addition, some notion of their length. A map, 
as we have seen, is a kind of picture, and the size of this 
picture must bear a certain relation to the size of the object 
represented. This relation or proportion is called the 
scale of the map. If a map is said to have a scale of one 
inch to the mile, it is simply meant that a mile measured 
along the ground is represented by an inch measured on 
the map ; or a square mile of country is represented by a 
square inch on the map, and so on. Most of the wonderfully 
accurate maps of the Ordnance Survey are constructed on 



PHYSIOGRAPHY. 



[chap. 



this scale of one inch to the mile. In other words, in the 
map of a given district, the distance between any two points 
is o"g"j"6-oth of the actual distance, since there are 63,360 
inches in one statute mile. The fraction which denotes the 
ratio of the two distances is sometimes termed the represen- 
tative fi'action. A map of the Thames on the one-inch scale 
would extend to a length of about 120 inches, since the 



Fig. 3. — Hili-shadias; by means of hachnres. 



greatest width of the basin of the river from east to west is 
about 120 miles. Maps on a scale even much greater than 
this are occasionally constructed. The Ordnance Survey, 
for example, issues county maps on a scale of six inches to 
a mile, the representative fraction being here j^p^g Q. But 
it is evident, from the size of a page of this book, that 
our map must be on a very much smaller scale; in fact,, 
a mile of ground is represented there by the fourteenth part 
of an inch. 



I.] THE THAMES. • 13 

In most maps, except those on an extremely small scale, 
an attempt is made to show something of the general 
features of the ground, especially whether the country is 
hilly or not. This is commonly effected by a system of 
hill-shading, such as that represented in Fig. 3.^ If the 
ground is steep, the lines, or hachures, are drawn thick and 
close together, so that the hilly spots become dark ; if the 
ground is tolerably level, the lines are thinner and farther 
apart, and the general appearance of the map is conse- 
quently lighter. Such a system of shading, however effec- 
tive by its combination of light and shade, shows in most 
cases merely that one part of the country is higher or 
lower than another, without enabling us to judge how 
much higher or how much lower. But in very accurate 
maps, such as those employed for military purposes, a 
definite scale of shade is often used. The same object 
may, however, be attained by an entirely different system, 
Such as that used in the map of the Thames forming 
Plate I. 

It will be observed that, instead of hill-shading, a number 
of curved lines have been traced over the map, giving it a 
peculiar character. These curves are called contour- Ihies, 
and their meaning is extremely simple. Suppose the valley 
of the Thames were flooded with water, and that this water 
could be dammed in or prevented from escaping, by a 
wall built across the mouth of the valley. If sufficient 
water covered the ground to stand 100 feet above the 
level of the sea, the surface of the water would form a 

1 This figure is copied from part of sheet 59 of the one-inch Ordnance 
map of England and Wales, and shows the physical characters of a 
district in North Wales. The portion of the map here copied is a 
square measuring two inches along each side, so that the total area 
represented in Fig. 3 amounts to four square miles. 



'4 



PHYSIOGRAPHY. 



[chap. 



plane, and its margin would trace a line, winding round 
every hill and up every valley, at a height of exactly loo feet. 
Such a line has been traced on our map, and being the first 
of the series of curves, it is numbered i. This is conse- 
quently the loo-feei contou7'-line. The second line, No. 2, 
is drawn at a height of 200 feet above the sea-le\^el, and 
therefore represents the margin of a body of water standing 
in the Thames Valley 200 feet above the sea. In like 
manner a succession of these contour-lines has been drawn. 




Fig. 4. — Contour-lines round a hill, 

each at a distance of 100 feet from the next one below, just 
as though the flood had risen in the valley and stopped at 
every 100 feet to leave its mark around its margin. It is 
evident that a system of such lines conveys a far better 
notion of the character of the ground than can be obtained 
from ordinary hill-shading. Where the ground is very steep, 
the contour-lines run close together ; where very flat, they 
stand far apart. The relation between the contour lines and 
the form of the ground is clearly shown in Fig. 4. In the 
upper part of this figure, a hill is represented by contour- 



1.] THE THAMES. 15 

lines ; and supposing this hill were cut through, along the 
line A B, it would give such an outline as that drawn in 
the lower part of the figure, the corresponding points in 
the plan and the section being connected by broken lines. 

Inspection of the map in Plate I. shows, as might have 
been expected, that the river there represented flows 
from high ground to low ground. In fact, if the reader 
were to travel up the Thames, by walking along its banks, 
he would find himself continually going up-hill. Between 
Thames Head and London Bridge, a distance of about 
170 miles, as measured along the winding course of the 
river, there is a difference of level of 370 feet. We 
have seen that the head of the navigation is at Lechlade, 
about 146 miles from London ; between Lechlade and 
London the river has a total fall of about 250 feet ; and as 
the descent is tolerably uniform, it may be taken at an 
average of 21 inches per m.ile. At Teddington, where the 
river ceases to be tidal, the ordinary summer-level of low 
water is 16J feet above low-water level at London Bridge; 
and the fall of the bed of the river below Teddington is 
nearly a foot in every mile. The rapidity with which a river 
flows will of course depend upon the amount of slope in 
its bed ; where the fall is great, the stream is rapid ; where 
small, the stream is slow. The bed of the Thames, for- 
tunately, is tolerably uniform in its descent, so that the 
stream is free from rapids. 

What has been said of the Thames is equally true of any 
one of its tributary streams : the source is always higher 
than the mouth. It is seen from the contour-lines on the 
map that, if we travel along any of the little rivers which 
open into the Thames upon its left bank, we go up-hill in 
passing from south to north ; if we travel along any of the 
streams on the opposite side of the river, we go up-hill in 



i6 



PHYSIOGRAPHY. 



[chap. 



passing southwards. As a consequence of 
all this, it follows that the tract of country 
drained by the Thames and its tributaries 
must be bounded on at least three sides— 
the west, the north, and the south — by com- 
paratively high ground. It thus forms a 
shallow depression, with an outlet to the 
east through which the river flows out to 
sea. Such a depression is known as a 
river-basin, and the country through which 
the Thames and its tributaries flow is conse- 
quently called the Thames basin ; while the 
deepest part of the basin, that in which the 
main stream flows, is termed the Thames 
Valley. The basin of the Thames, depicted 
in Plate I., includes a very large tract, ex- 
tending over 6,1 60 square miles; the Thames, 
in fact, drains more than one-seventh of all 
England. 

Perhaps the term "basin," just used, is 
one which is likely to mislead unless properly 
qualified. It is true that, if you go north 
from any part of the valley of the Thames, 
you find yourself sooner or later travelling 
up-hill, and therefore reach higher ground 
than that through which the river flows ; if 
you travel southwards, you do the same thing ; 
while towards the west, the assent is not less 
marked. The river then really does occupy 
a hollow, inclosed on three sides by high 
ground. But it must be borne in mind that 
this hollow is nothing like the deep hollow 
associated with our ordinary notion of a 



I.] THE THAMES. 17 

basin ; it is, in fact, so slight a depression that it would 
perhaps be better to speak of the "dish" of the Thames 
rather than of its " basin." In Fig. 5 the undulating line 
indicates the general contour of the surface of the country 
along a north-and-south line drawn across the basin of the 
Thames, from the Chiltern Hills on the north, to the North 
Downs on the south. The very gentle curvatures of this 
line show the extreme shallowness of the so-called basin ; 
and also show the irregularities in the form of the ground. 
Although the opposite north and south hills may attain to 
a height of several hundred feet above the river, yet the 
distance between them is so great, amounting to some fifty 
or sixty miles, that the rise from the river to their summits 
would be almost inappreciable in a diagram brought within 
the limits of this page. Hence it is a common practice in 
constructing such diagrams to draw the heights to a scale 
many times greater than that used for the lengths. This has, 
in fact, been done in Fig. 5, where the vertical is as much as 
twelve times greater than the horizontal scale. Without such 
exaggeration, the surface of the country would appear in 
a small diagram to be almost flat ; and even with it, the 
extreme shallowness of the Thames basin is strikingly ap- 
parent. There is clearly no harm in the practice of drawing 
diagrams on this principle, provided that the exaggeration 
of one dimension is always acknowledged by the draughts- 
man, and borne in mind by the reader. Great misconception, 
however, constantly arises from mistaking these intentionally 
distorted diagrams for true figures. 

To the north-west of London, the margin of the Thames 
basin is formed, in part, by a line of low hills called the 
Chiltern Hills ; and, on the south of London, there is an- 
other series known as the North Downs ; while, if we go far 
enough to the west in the Thames basin, we come to a still 

c 



i8 PHYSIOGRAPHY. [chap. 

higher country forming the Cotteswold Hills in Gloucester- 
shire. Suppose the reader were to ascend one of these 
ranges of hills, say the Cotteswolds, on the west. As he 
v/ent up, he would meet with many little streams which are 
flowing down to feed the affluents of the Thames. But 
having reached the summit, and walked on in the same 
direction, he would soon begin to go down-hill, and then 
meet fresh streams running in an opposite direction to 
those he had left. These new streams cannot possibly flow 
into the Thames, for to do that the water would have to run 
up-hill. By following these streams, however, he would find 
that they ultimately flow into a river entirely distinct from 
the Thames ; thus, on the other side of the Cotteswolds, the 
streams find their way, sooner or later, into the Severn. On 
crossing these hills, then, we have passed from the basin of 
the Thames into that of the Severn. The high land which 
forms the divisional line between two contiguous river-basins 
is called the water-parting. 

Instead of "water-parting," some writers employ the 
term watershed ; but although the two words originally 
meant precisely the same thing, the latter has become 
rather ambiguous. " Watershed " is a word which has been 
borrowed from German geographers. The verb scheiden 
signifies " to separate," and die Wasserscheide is simply the 
" water-separation " or the " parting of the waters " — the old 
Divortium aquarurn. But many writers, looking at the 
common meaning of the English verb "to shed" have 
used the term " watershed " to denote the surface from 
which the waters are shed, or the slope along which they 
flow ; hence it is not uncommon to hear such expressions 
as " the crest of the watershed." To avoid this double u-se 
of the word "watershed," the term " water-parting " has 
been introduced as the English equivalent of the German 



{] THE THAMES. 19 

Wasserscheide, or the boundary-line between two adjacent 
river-systems. Such a line was called by Professor Phillips 
"the summit of drainage " ; and in the north of England, 
where it often separates one estate from another, it is known 
as " the heaven-water boundary." To avoid all ambiguity, 
it is perhaps best to set aside the original meaning of 
"watershed," and employ the term to denote the slope 
along which the water flows, while the expression " water- 
parting " is employed for the summit of this slope. Thus 
the ridge of a roof is the water-parting ; and the slates or 
tiles on each side, down which the water drains, will form 
the watershed. It must be remembered, however, that the 
water-parting is not necessarily the summit of a range of 
hills, like the ridge of a roof. Frequently, indeed, the 
ground is only relatively high ; but the water easily finds 
the slope, however small, and runs down it, thus showing at 
once the direction of the water-parting. 

A little consideration will show that water-partings may 
be drawn on a map of any country, so as to divide the 
entire region into a series of river-basins. Plate II. is a 
map of Great Britain and Ireland thus completely divided 
into river-basins, separated one from another by water- 
partings, which are indicated by dotted lines. All the 
rivers which empty themselves into the sea on the eastern 
side of Britain may, in this way, be separated from those 
which run into the western seas, and both systems may be 
separated from the southern rivers which open into the 
English Channel : the northern drainage is insignificant. 
We thus obtain the general water-parting of Great Britain, 
distinguished in the map by a line of red dots. This is a 
sinuous line running from near John-o'-Groat's House, 
through Scotland and the north of England, down the Peak 
of Derbyshire and through the Midland counties, till it gets 



20 PHYSIOGRAPHY. [chap. i. 

as far south as Salisbury Plain, Such a line divides the 
western drainage of the country from its eastern drainage. 
At Salisbury Plain the line splits into two branches, one 
stretching to the east coast and terminating somewhere 
about Dover, the other striking to the west coast and ter- 
minating at the Land's End. To the south of this great 
east-and-west line, all the rivers flow into the English 
Channel. This three-branched line consequently represents 
the main water-parting of the country ; it is in fact the 
general high-level line of Britain, though it has no direct 
relation to the mountain-systems of the country. The main 
water-partings of Ireland are also indicated on the map, the 
rivers being grouped in four great systems, which drain to the 
north, south, east, and west. 

We shall restrict our attention, for the present, to one of 
these river -basins of England — the basin of the Thames — 
and endeavour to extract from its study as much instruction 
as possible. In several of the succeeding chapters we shall 
therefore inquire how this basin is fed with water, by what 
means it has received its present shape, and what has been 
its history in the past. Even the first question — how the 
basin is fed — suggests prolific material for study. It is true 
we have, in the present chapter, traced the Thames to its 
head-waters, but it must not for a moment be supposed that 
by doing this we have yet reached its real origin. The 
streams and springs from which a river is popularly said 
to take its rise are in truth only its proximate sources, and 
the ultimate source is to be sought elsewhere. In seeking that 
source, the inquiry may fitly be commenced by examining 
more closely into the nature and origin of springs. 




^ Hr -^ '^ 



S 






Fold-out 
Placeholde 



^ :^ 



This fold-out is being digitized, and will be 

future date. 



CHAPTER II. 

SPRINGS. 

Mark what happens when a heavy shower of rain falls upon 
dry ground. If the ground be formed of hard and solid 
rock, such as granite, the rain, after wetting the surface, runs 
off in all directions ; some finding its way to the nearest 
streamlet, whence it flows sooner or later into a river, and 
some finding lodgment in little hollows of the rock, where 
it collects in pools which are slowly dried up by wind and 
sunshine. But if the ground, instead of being hard like 
granite, is soft and porous like sand or chalk, the water 
will then sink into its substance, and may even pass out of 
sight before the surface of the thirsty soil is thoroughly 
wetted. Rocks which thus allow water to filter through 
them are said to be permeable^ while those which refuse to 
allow the water to soak in are said to be iinpenneable : 
a bed of sand, for example, is permeable ; a bed of clay 
impermeable. 

It is by no means necesssary, however, that a rock, in 
order to be porous and permeable, should be either soft 
like chalk, or loose like sand. Take for instance a sand- 
stone, or a hard limestone : these rocks are sufticiently 
coherent to form durable building stones, vet porous enough, 



22 PHYSIOGRAPHY. [chap. 

in most cases, to allow water to drain more or less freely 
through them. The particles of which the rock is made up 
are themselves impermeable, but they are so built together that 
little spaces, or interstices, are generally left between the 
individual particles, and the result is the formation of a 
rock which, hard as it may be, presents a texture approxi- 
mately like that of a sponge. The water trickles between 
the particles of such a rock, and thus gradually soaks 
through its mass. Close in grain as the rock may appear 
to the eye, it is nevertheless capable, in most cases, of 
absorbing water; and, hence, stone when freshly taken from 
the quarry usually holds moisture, known to the workman 
as " quarry water." Even when a rock offers too close a 
texture to admit moisture freely, it commonly happens that 
it is more or less fissured ; and the water which falls upon 
the rock then dribbles through the little cracks, and thus 
gains ready access to subterranean channels, much in the 
same way as it would if the rock were of open texture. 

After a good deal of rain has fallen upon a porous rock, 
its pores become choked with water, and the rock at last 
gets saturated, like a piece of sugar which has been dipped 
for a few moments into a cup of tea. If more rain now 
fall upon the rock, it can no longer be sucked in and 
retained, but will flow off the damp surface, just as it would 
from the surface of an impermeable rock. 

Suppose a layer of a porous substance to rest upon a bed 
of comparatively impervious rock, and it is easy enough to 
see what, under such circumstances, will become of the rain 
which falls upon the surface. Let Fig. 6 illustrate such a 
case. Here the dotted part of the figure A B C D, repre- 
sents a permeable rock, say beds of sand, whilst the lower 
shaded portion, C D E F, indicates an impermeable rock, 
say a stiff clay. It is supposed, in a figure such as this, that 



II.] 



SPRINGS. 



23 



the rocks in question have been cut through so as to expose 
clean-cut surfaces, and hence such diagrams, which are 
constantly used in writings on the structure of the earth, 
are termed sections. Natural sections are frequently exposed 
in river-beds, sea-cliffs and inland valleys ; whilst artificial 
sections are seen in wells and shafts, in mines and quarries, 
and especially in railway-cuttings. A good general notion 
of the character of the rocks forming a given country may 




Fig. 6. — Formation of a spring. 

often be gained, during a railway journey, by observing the 
cuttings along the line.' 

1 Some good geological sections may be seen in railway cuttings in 
the neighbourhood of London. Thus, in travelling by the North 
Western Railway to Watford, the passenger may observe the London 
clay ; and, further on, the Lower London Tertiaries overlying the 
chalk. On the Great Western Railway, the route to Maidenhead passes 
over the brick-earth and gravel which overlie the London Clay, and, at 
Windsor, the chalk is exposed, but not on the railway. To the south 
of London, some good sections may be seen in the cuttings on the North 
Kent Railway in the neighbourhood of Lewisham and Charlton, where 
the sandy deposits, known as the Thanet Sands, are exposed. A 
journey to Reigate, on the Brighton line, carries the passenger over the 
London clay, the chalk, and the beds below the chalk known as 
Greensand. For other sections see Whitaker's Guide to the Geology oj 
London. 



24 PHYSIOGRAPHY. [chap. 

It is clear that when rain falls upon the surface A B, it 
vvili be readily absorbed, at least if the sandy rock be dry, 
and will gradually sink lower and lower until it reaches the 
bottom of the upper bed C D. Here it comes in contact 
with the surface of the clay, and, as the clay refuses to 
absorb the water, its downward course is arrested. Should 
the surface of the clay present irregularities, the water which 
has percolated through the sandy bed will lodge in the 
hollows, as at G. But when such cavities have become 
filled, the water with which the rock is charged will flow over 
them, and continue to run down in whatever direction the 
rocks may chance to slope. 

It rarely happens that the successive layers of rock, or as 
they are technically called strata,^ exposed in any given 
section are perfectly horizontal, or spread out with flat 
surfaces, like the surface of a piece of still water. Generally 
the beds slope or incline in a definite direction, and this 
slope is technically termed the dip. If then we read in a 
scientific description of a given section that " the strata dip 
30° S.W." it means simply that the layers of rock slant in a 
south-westerly direction, and make an angle of thuty 
degrees with a perfectly horizontal surface. Thus, in the 
diagram the dip is shown by the general direction of the 
line CD, and its amount may be measured by the inclination 
of this line to the horizmi ; that is, by the angle which the 
line C D makes with the top or the bottom edge of the page, 
when these edges are horizontal. Now when the water, 
having percolated through the sandy rock A B C D, has 
reached the junction represented in section by C D, it flows 
down this plane in the direction of the dip, and escapes at 

^ Stratum (plural strata) from the Latin stratum, signifying that 
wJiich is extended or spread out. 



ij.J SPRINGS. ■ 25 

the first outlet, as at D. Such a flow of water thrown out 
from a rock constitutes a spring. 

Springs of this simple character, which irssue at the 
junction of permeable and impermeable strata, are extremely 
common. If the reader who lives in London will take the 
trouble to walk across Hampstead Heath soon after rain 
has fallen, he can observe for himself exactly the condition 
of things which has just been described. The highest 
ground at Hampstead rises to about the height of St. 
Paul's, or rather more than 400 feet above the sea-level, 
and consists of loose sand which forms irregular ground 
overgrown with ferns and gorse. This sand is similar to 
the sand which is spread out in large mass over the widely- 
separated district of Bagshot Heath in Surrey, whence it is 
termed Bagshot sand. It is, however, only the highest 
ground on Hampstead Heath that is formed of such sand, 
which indeed does not attain, in this locality, more than 
eighty feet in thickness. Beneath the sand is the same 
stiff brown clay which underlies the whole metropolis, and is 
consequently known as the London clay. Hampstead Heath 
is therefore formed of London clay capped by Lower 
Bagshot sand, as shown in Fig. 7. This is taken from a 
section drawn by the Geological Survey, but in order to 
bring the section within the width of the page, and yet pre- 
serve its characters, the vertical scale has been exaggerated. 
On a natural scale the heights in the figure would require to 
be reduced by one half, and the capping of 3and would 
consequently appear of insignilicant thickness. Where one 
kind of soil ends and tiie other kind begins, is pretty sharply 
mapped out by the effect of rain. Any one who walks over 
the Heaih after a shower will not fail to observe that 
the sandy soil remains almost perfectly dry, the rain having 
been at once sucked in, while the clay, only a few yards off, 



16 PHYSIOGRAPHY, [chap. 

remains wet and sticky. The water absorbed by the sands 
drains through them until it reaches the clay beneath, when 
it either oozes out in an irregular sheet of water, or is dis- 
charged through definite channels as springs. The line 
of springs consequently serves to mark the junction between 
the two kinds of soil. As the surface of the clay receives 
the drainage of the sands it is constantly kept wet, and 
thus forms swampy ground, often marked by the growth of 
rushes. It sometimes happens that the upper part of the 
London clay is more or less sandy ; and, consequently, it is 
not always easy to say precisely where the London clay 
ends and the Bagshot sands begin. The springs are. 




Fig. 7. — Section of Hampstead Heath. Vertical scale twice the horizontal scale. 

however, thrown out as soon as the water reaches an 
impervious bed. 

The sands of the Bagshot series as exposed on Hampstead 
Heath are commonly, of yellov/ and brownish colours. 
These colours are due. to the presence of a peculiar com- 
pound of iron, oxygen,^ and water, known to chemists 
as hyd?viis peroxide of iron or ferric hydrate. As the rain- 
water slowly filters through these ferruginous sands it dis- 
solves more or less of this iron-compound, and carrying 
it off in a soluble form, acquires medicinal properties. Thus 
the spring in Well-walk at Hampstead is locally valued as a 
tonic ; while its inky taste, and the foxy-red sediment 

1 For description of the gas called oxygen, see pp. 78, 79, 80. 



II.] SPRINGS. ■ 27 

which it deposits on standing, sufficiently attest the presence 
of iron: it is in fact a Chalybeate^ spring. This example 
sufficiently illustrates the origin of mineral springs. The 
saline and other substances which they contain, and on 
which their peculiar properties depend, are generally 
dissolved out of the rocks through which the water flows. 
In the basin of the Thames, mineral springs are by no 
means uncommon, some being chalybeate, others sul- 
phureous, and others again saline. This is a subject, 
however, which will be considered by and by. 

What has been said, with respect to the structure of 
the high ground at Hampstead, applies equally to 
that at Highgate, and at Harrow. In each of these 
localities, an isolated patch of Bagshot sand rests upon 
London clay. And, coming down to the lower ground on 
which the greater part of the metropolis is seated, a very 
similar condition of things is met with ; that is to say, a 
layer of highly pervious material is spread over a rock well 
nigh impervious. The porous material, however, instead of 
being Bagshot sand, is here a bed of gravel, varying from 
ten to twenty feet in thickness, and resting upon the 
London clay. When the rain has soaked into this gravel, 
it is held up by the clay below, and is thus preserved in a 
great underground reservoir, which offers a never-failing 
source of supply to the shallow wells which were formerly 
sunk in great numbers throughout London. Here and 
there, a little valley cuts down through the gravel to the un- 
derlying clay ; and, the natural reservoir being thus tapped, 
a spring of water flows out at the junction. Such is the 
origin of the springs which gave names to Clerkenwell, 

' Chalybeate from x°'^'"'^> chalups, steel or hardened iron, a Greek 
word derived from tlie people called Chalybes, who dwelt on the 
southern shore of the Black Sea, and were famous iron-workers. 



28 PHYSIOGRAPHY. [chap. 

Holywell, Bagnigge Wells, and other localities. Wells 
supplied from this gravel constituted for centuries the sole 
water-supply of the metropolis, and Professor Prestwich has 
well pointed out how " the early growth of London followed 
unerringly the direction of this bed of gravel." ^ As long 
as this was the case, settlement was quite impossible where 
the gravel was absent and the clay exposed. Indeed, it 
was not until an independent source of water was supplied 
by the great water-companies, that a population was 
estabhshed on the clay-districts of Camden Town, St. John's 
Wood, Notting Hill, &c. 

Along the banks of the Thames and its tributary streams 
there is a bed of valley-gravel, at a lower level than that to 
which reference has just been made. This low-lying gravel 
also forms a source of water-supply to shallow wells, and has 
determined the site of many centres of population. Thus, 
Westminster, Battersea, Hammersmith, Brentford, Eton, 
Maidenhead, and many other towns up the Thames, were 
originally dependent upon this source for water. 

From the cases hitherto considered, namely, those in which 
a porous rock rests upon one that is not sensibly porous, it 
is desirable to advance to the case in which the porous 
material is not only supported, but is also covered, by an 
impermeable stratum ; the pervious substance being thus 
inclosed between two impervious beds, one forming its 
floor and the other its roof Thus, the sandy stratum B, 
in Fig. 8, is supported by one bed of clay, C, and covered 
by another, A. As long as the strata remain in the 
horizontal position here represented, the rain which falls 
upon the surface of A is effectually prevented from reaching 
tiie porous material B, save only through any cracks which 

' Anniversary Address to the Geological Society 1872, Quart. Journ. 
Geol. Soc. vol. xxviii. No. 110. p. liii. 



n.l 



SPRINGS. 



29 



may happen to run through the upper bed. Though the 
material of B may be as porous as a sponge, not a drop of 
water can reach it, as long as the waterproof roof remains 
sound. But the case is extremely different when the beds 
are inclined, as represented in Fig. 9. Here are three 
beds, in the same order as those previously described, but 
dipping at a slight angle. The porous bed B is exposed 
at the surface, or as a geologist would say " crops out." 




Rain 



Fig 8. — Horizontal permeable and impermeable strata 

falling upon the ground A B C is thrown off by 
the two clay beds A and C, but is absorbed by the out- 
crop, or exposed surface, of the sandy stratum B. This 
absorbed water, whether directly falling upon B, or drained 
off from A, runs down in the direction of the dip until 




i'lG. 9. — Inclined permeable and impermeable strata. 

it reaches an oudet, whence it issues as a spring. If a 
valley should cut through the beds, and have its bottom 
below the water-level, springs will be thrown out along 
the sides of the valley, as at D. 

In following the course of a set of strata, it is no un- 
common thing for the geologist to find that they come 



3° 



PHYSIOGRAPHY. 



[chap. 



abruptly to an end, that their continuity is suddenly broken, 
and that one set of beds abuts upon another along a sharply- 
defined plane. The beds have, indeed, been fractured, and 
have slid one over another. Such a fracture, accompanied 
by displacement of the strata, constitutes what geologists call 
z. fault. Thus the set of beds represented in Fig. lo have 
been broken along a plane represented in section by the 
line D E; and, though once continuous, are now dislocated ; 
the bed A having been thrown down to A', the bed B 
having slipped to the position B', and the bed C to C 
The drainage received by the surface of the porous stratum 
B, will flow down until it reaches the fault, where it will be 




Fig. 10. —Effect of a fault on the position of a spring or well. 

prevented from escaping by the clay wall of A'. If there- 
fore a bore-hole be put down to F, the water which has 
percolated through the bed down to this point will be 
forced upwards by the pressure of the water in the sur- 
rounding rock, and will therefore rise in the hole to nearly 
the level which it occupies in the bed B. Or, in the absence 
of a bore-hole, the water will escape from the saturated 
bed B, by oozing out at the surface, near the junction of 
the adjacent strata. It is obvious, from this illustration, 
that faults must be of great importance in determining the 
position of springs and wells. 



II.] 



SPRINGS. 



31 



It frequently happens that the beds of rock, instead ot 
ha /nig a uniform dip, slope first downwards and then 
upwards, so as to assume a basin-like form, such as that 
shown in Fig. 11. Here it is seen that the strata on opposite 
sides slope in opposite directions, as indicated by the 
arrows. Rain falling upon the exposed edges of the 
porous rock B B, will be readily absorbed, and will per- 
colate through the pervious material until it reaches the 
bottom of the trough, where it will accumulate, and of 
course be accessible to the boring-rod. If, therefore, a 
bore-hole be put down through the im.pervious bed A, 
it will tap this reserv^oir of water, and the liquid will then 
rise to a height dependent on the level of the water in 




Fig. II. — Diagrammatic seciion across the London basin. 



the bed B B. The laws which regulate the flow of watei 
underground are precisely the same as those which regulate 
its flow above ground. The water pent up in the bed B B 
will therefore rush up the tube, and tend to find its own 
level. 

This arrangement of strata in Fig. 11, may be taken 
to represent, roughly, the disposition of the rocks beneath 
London. The beds in that area have been thrown into 
a trough-like form, and have thus produced what is com- 
monly known as the Lo7idon basin} Yet it must not for 

^ The London basin, or area around the metropolis, forms only a 
small portion of the Thames basin. The two are to be kept carefully 
distinct 



32 PHYSIOGRAPHY. [chap 

a moment be supposed that they lie in a deep hollow 
anything like that of an ordinary domestic basin, or even 
like that represented by the curves in the figure. It is 
true the rocks to the north and south of London slope 
gently inwards, and thus produce a depression, but it is 
a depression of the very shallowest kind. The inclination 
is indeed so gentle that it can scarcely be shown in a 
diagram brought within the compass of a page of this 
book. Hence most diagrammatic sections, as already ex- 
plained (p. 17), in the description of the basin of Thames, 
are necessarily exaggerated, and false notions have thus got 
abroad as to the nature of the London basin. It is only 
when true sections are drawn, having the vertical heights 
represented on the same scale as that of the horizontal dis- 
tances, that its extremely gentle concavity becomes apparent. 
With this caution, we may proceed to examine more 
closely the section in Fig. 11. This may be taken to 
represent an exaggerated section across London, from 
north to south ; A representing the London clay, and B 
indicating the position of certain underlying rocks which 
geologists are in the habit of calling the Lower London 
Tertiaries. These lower strata are extremely variable, but 
in the neighbourhood of London they consist for the most 
part of sandy deposits, and are therefore highly permeable. 
Thin beds of clay, spread out at different levels in the 
sands, serve to retain the water ; and the supply, thus stored 
up, has been utilized by borings carried down through the 
overlying London clay. Such borings were frequently 
made in the early part of this century, and an abundant 
supply of water was obtained from depths varying from 80 
to 140 feet. So numerous, however, were these wells, that 
a great drain was thrown upon the water-yielding power of 
the strata, and the supply ultimately became unequal to the 



ti.] SPRINGS. 33 

demand. Of late years, therefore, most of the deep wells 
in London have been carried yet lower, passing in fact into 
the great mass of chalk which underlies the Lower Ter- 
tiaries in the position represented by C C in Fig. ii. The 
water is obtained partly from the saturated chalk and partly 
from fissures, the latter being in this case the more im- 
portant source. As the position of these irregular cavities 
cannot be predicted, it is clearly impossible to foretell the 
depth at which stores of water will be found in the chalk. 

When a deep boring is made through the London clay 
down to the Lower Tertiary sands, or still deeper, into the 
chalk, the water tends to rise in the tube, and may even 
reach the surface and overflow. If the point from which 
the water is tapped be in low ground, as would be the case 
with the boring D in the hollow of the London basin, it is 
necessarily at a much lower level than that of the outcrop 
of the beds along the margin of the trough, as at C C. 
The water is consequently forced up the tube by the pres- 
sure of the liquid with which the water-bearing bed is 
charged. When this pressure is sufficient to cause tlie 
water to flow over the surface of the bore- hole, it produces 
what is termed an "Artesian Well." This name, however, 
is now commonly extended to other wells in which the 
water, without overflowing, yet rises to a sufficient height to 
be economically employed. Artesian wells are of great 
antiquity in the East ; in North- Western Europe they were 
first constructed in the province of Artois, in France, 
whence the name A?'fesian. 

In the London basin there are numerous wells supplied 
on the Artesian principle. Thus the fountains in Trafalgar 
Square are fed with water from an Artesian well, which 
penetrates the chalk to a depth of about 390 feet beneath 
the '•'irface. It would be curic^us to trace the history of a 

D 



34 PHYSIOGRAPHY. [chap. 

drop of water brought up by these fountains. It may have 
fallen originally as rain upon the chalk hills around London, 
perhaps twenty miles off, and after slowly trickling through a 
long, dark underground course, where it was pent up under 
pressure, ultimately found relief in the bore-hole of the 
Artesian well. 

London and Paris are situated under very similar 
geological conditions, and what is said with respect to the 
deep wells of one city applies with but little modification to 
those of the other. In the Paris basin, the Artesian system 
has been very largely carried out, and many of the borings 
have reached extraordinary depths. Thus at Crenelle, a 
suburb of the south-western part of Paris, there is a famous 
Artesian well, 1,798 feet deep, fed by the rains which fall on 
the permeable strata of Champagne at a distance of 100 
miles from Paris. Several other borings have since been 
carried to much greater depths than that reached by the 
well at Crenelle, 

Having explained the origin of ordinary springs and the 
nature of Artesian wells — which in truth are merely artificial 
springs — it is now time to return to the study of the Thames 
basin, the springs of which are the immediate source of all 
the fresh water in the river, except the small portion derived 
from the rain which falls directly into it. 

The alternations of pervious and impervious strata 
which constitute that district, present all the necessary 
conditions for an abundant supply of springs yielding 
excellent water. " In the whole course of our experience," 
said the Rivers' Pollution Commissioners, in their sixth 
Report, " we have found no catchment-basin ' so rich in 

^ The caicJiment-bctsin is a term applied to all that part of a river- 
basin from which rain is collected, and from which therefore the river 
is fed (p. 145). 



II.] SPRINGS. 35 

springs of the finest drinking-water as that of the Thames." 
The basin of the Thames, as we have seen above (p. 15), 
has an area of more than 6,000 square miles, and rather 
more than half of this area is formed of porous soil, 
resting upon water-bearing strata, whilst the remainder 
consists of clayey soil. The pervious ground, which ab- 
sorbs the rain, is, for the most part, under cultivation ; the 
impervious ground, which throws off. the rain, is mostly 
permanent meadow-land and pasture. 

In describing the springs in the upper part of the Thames 
basin, or those springs which rise above Oxford, we may 
be guided by the careful observations which have been 
made for many years by Mr. Bravender, of Cirencester.^ 
The source of the river Churn, or the extreme western 
tributary of the Thames, is known as " The Seven Springs," 
and is situated about four miles from Cheltenham on the 
road to Cirencester. It has been estimated that these 
springs yield on an average about 150,000 gallons of water 
daily. The Seven Springs are thrown out from clayey beds 
which belong to the series of rocks known to geologists as 
Lias ; ^ the water having been absorbed by the overlying 
loose limestones which are technically called Inferior Oolite^ 
and which contain cavities that serve as excellent subter- 
ranean reservoirs for storage of water. The term " oolite," 
which has just been used, is applied by geologists to a large 
series of rocks, which occupy a definite position in the scale of 
strata and include many limestones which are composed of 
pecuharly rounded grains, that give the rock somewhat the 
appearance of the roe of a fish, whence the word oolite (&>6v, 

^ See his evidence before the Royal Commissions on Water Supply 
and Rivers' Pollution. 

^ The geological structure of the Thames basin is shown in Plate 
v., and will be explaiaed in detail in Chapter XVII, 

D 3 



33 PHYSIOGRAPHY. [chap. 

ail egg). The lower part of this ooHtic series, which rests 
directly on the Lias, is termed, from its position, the Inferior 
Oc\ite. It is from some of the beds of this formation that 
the Seven Springs are fed. Rain falling on the rulzbly lime- 
stones., the sandstones, and loose sands of the Inferior 
Oolite, percolates downwards, through numerous cracks and 
crannies, until it reaclnes the impermeable clays of the Lias, 
when it gushes forth wherever a channel offers itself. About 
one-third of all the rain that falls upon the inferior oolites is 
returned in the form of springs. So porous and open- 
jointed are some of the rocks of this series that, where they 
form the channel of the river, a considerable quantity 
of the water which flows over them is absorbed, and the 
volume of the stream may thus be diminished to a serious 
extent. 

The Syreford Spring at the head of the river Colne — 
one of the streams in the upper basin of the Thames — 
has an origin similar to that of the Seven Springs. The 
Syreford source yields daily from three to four millions 
of gallons of water. This water has been absorbed by the 
Inferior Oolites and is thrown out by the underlying Lias 
clays. 

Many of the springs in the head-waters of the Thames 
take their origin in the water-bearing limestone beds known 
as the Great Oolite — a set of rocks separated from the 
inferior oolite by the so-called Fuller's earth. This Fuller's 
earth forms a thick bed of clay, which retains the water that 
reaches it, in enormous quantity, by percolation through the 
porous limestones and sands of the great oolite series. 
Thus, the famous well at " Thames Head," the position 
of which is indicated on the map in Plate I., rises in this 
way from the surface of the Fuller's earth, and yields a vast 
volume of water which has been collected in the fissures of 



II.] SPRINGS. • 37 

the Great or Bath Oohte. Three milhon gallons of water are 
every day pumped up by the engine from a depth of 35 
feet into the summit-level of the Thames and Severn canal. 
This artificial withdrawal of v/ater to feed the canal has 
lowered the level of the water-bed in the surrounding 
country, so that the natural discharge at Thames Head is 
now contracted and takes place lower down the valley. 

About five miles south-east of the engine at Thames 
Head is the Boxwell Spring, which discharges into the 
Churn more than a million gallons daily. This large body 
of water is drawn off from the surface of the Fuller's earth ; 
and many other springs, such as the Ewen Spring and the 
Ampney Spring, have a like origin. 

Other streams tributary to the Thames take their rise in 
the springs of the porous sandy beds called Upper Green- 
sand^ which rest upon a stiff clay known as Gaiilt. In 
the lower part of the basin, or nearer to the mouth of 
the river, the Chalk becomes the great reservoir ; and it 
often happens, that water which has drained through the 
porous upper part of this formation, and made its way 
through fissures, is held up by the lower part of the chalk, 
which becomes stiff and retentive. The Kennet, which is 
one of the main feeders of the Thames, receives its water 
chiefly from the chalk downs near Hungerford and Marl- 
borough ; whilst the Coin collects a large body of water 
from the Hertfordshire chalk. A great part of the Nevv 
River water too is drawn from the chalk. Finally, as a 
source of water in the Thames basin, the Bagshot Sands, 
already noticed, must not be neglected. Thus, the Loddon, 
a tributary from the south, which falls into the Thames 
about five miles below Reading, derives part of its water 
from the Bagshot series. 

As the Thames pursues its course, it receives supplies of 



3S PHYSIOGRAPHY. [CH. il. 

water not only from its tributary streams, but from springs 
gushing up in the very bed of the river. These springs 
are in some cases of enormous magnitude, especiaUy 
between Wallingford and Reading, and thus swell the 
volume of the river to no inconsiderable extent. 

Enough has been said, in this chapter, respecting the 
nature and origin of springs, to shov/ that all such sources 
of water owe their origin, directly or indirectly, to the 
rain which falls upon the collecting ground, and linds its 
way through the pores and cracks of the rocks beneath. 
Proximately, the source of the Thames and other rivers is to 
be found in springs ; but, ultimately, it must be traced to 
rain. It is true the springs feed the river, but it is the rain 
that feeds the springs. It will therefore be necessary, in the 
next chapter, to study the formation of rain and kindred 
phenomena. 



CHAPTER III. 

RAIN AND DEW. 

In travelling by steamer, it often happens that on going to 
that side of the boat towards which the wind is blowing 
the passenger suddenly finds himself in a shower of fine 
rain. This artificial shower is produced by the steam which 
issues from the waste-pipe being cooled down by contact 
with the surrounding cold air until it is condensed in the 
shape of drops of liquid. Every natural shower of rain 
is produced by a process of condensation similar to this, 
but carried on in the higher regions of the atmosphere. 

It is instructive to observe the dense clouds of steam 
which roll forth from the spout of a kettle of boiling water, 
or from the escape-pipe of a steam-engine. In most cases, 
nothing can be seen close to the point from v/hich the 
vapour issues, and it is only at some distance from this 
point that the white clouds first make their appearance. 
But, since that intervening space lies directly in the path 
of the issuing vapour, it is clear that it must be traversed 
by steam, though the eye fails to detect it. In fact, the 
steam, or watery vapour, when pure and uncondensed, is 
as transparent, as colourless, and as invisible as the air 
we breathe or the gas we burn. It is only when the vanonr 



40 PHYSIOGRAPHY. [chap. 

is partially condensed, and therefore ceases to be true 
vapour, that it appears in those cloud-like forms which are 
popularly called "steam." Could you look into the interior 
of a kettle or of a boiler from which clouds of so-called 
steam are issuing, you would see absolutely nothing in 
the space above the boiling water. It is only necessary to 
boil water in a glass vessel, such as a Florence flask, m 
order to observe that the steam remains invisible until 
exposed to some chilling influence, such as that of a body 
of cold air. 

More or less of this watery vapour or steam, in its 
invisible condition, is constantly present in the surrounding 
atmosphere. It rises into the air from every exposed piece 
of water, which is warmed by the solar heat, just as steam 
is generated from water which is artificially heated. Whether 
it be evolved rapidly, with formation of bubbles, as in 
the ordinary process of boiling, or slowly and quietly, 
as in the course of evaporation, the product is the same 
— namely, invisible watery vapour. But let the air thus 
charged with moisture be sufficiently cooled, and its 
burden of vapour, previously unseen, makes its appear- 
ance as cloud, or mist, or fog. And under certain atmo- 
spheric conditions, the condensation proceeds further, 
until the moisture ultimately falls to the earth in the 
shape of rain. Every one knows that if a cold object, 
such as a steel knife, be held in a cloud of steam, the 
surface rapidly becomes covered with drops of condensed 
water ; and the drops of water in a shower of rain have 
been generated by a similar process of condensation, carried 
on in nature. 

In most cases, the atmospheric moisture passes through 
the condition of visible cloud, or mist, before finally con- 
densing as rain. Yet it sometimes happens that rain falls 



III.] RAIN AND DEW. • 41 

from a clear and cloudless sky. By local refrigeration, after 
sunset, the vapour, invisibly diffused through the atmosphere, 
is condensed, at once, into excessively fine drops of Uquid 
water, forming the rain called serein. But such phenomena 
are rare ; and, as a rule, we may fairly expect the formation 
of rain to be preceded by that of cloud. 

Many opinions have been advanced to explain the 
precise condition in which water exists in a cloud. At 
one time it was comuionly supposed that a cloud is made 
up of a vast number of minute vesicles, or little watery 
bladders, which remain suspended in the air by reason of 
their small size and hollow structure. It appears probable, 
however, that the water is merely condensed in a very 
finely-divided state, its extremely minute particles remaining 
suspended in the surrounding moist air, as fine dust would 
do. Such particles have, indeed, been expressively called 
by Prof. Tyndall "water-dust." It is supposed that, in the 
ui)per regions of the atmosphere, the watery cloud-drops are 
frequently frozen into ice — a supposition strongly supported 
by the optical characters of certain clouds, which appear to 
be explicable only by the presence of a crystalline structure. 

When a current of warm air, laden with moisture, rises 
from the surface of the earth, and reaches the higher and 
colder regions, the uppermost portion of the ascending 
current deposits its moisture in visible form, and thus 
produces a cloud, supported at the top of an invisible 
column. If the temperature fall, or the course of the 
current be arrested, the cloud descends, and regaining 
the lower and warmer regions, returns to its original state 
of invisible vapour, and thus becomes dissipated. Observe 
the clouds of steam which issue from the chimney of a 
locomotive engine, and you will see that as they float away 
in the air they gradually disappear. The minure particles 



42 



PHYSIOGRAPHY. [chap. 



of water of which they are composed, in fact, are speedily 
converted into invisible vapour, and mingle with the atmo- 
sphere ] and the drier and hotter the air happens to be, the 
more rapidly does this change take place. 

Again, if a current of warm and moist air meet a colder 
current, its temperature is reduced, and more or less of its 
moisture is deposited. In this country, the south-west winds, 
having swept over the warm waters of the Atlantic, are 
charged with moisture, and are ready to deposit a portion of 
their freight whenever they are sufficiently chilled, as they 
may be, for example, by meeting a cold east wind. Hence 
south-west winds act as the chief rainbearers to our islands. 

So fantastic and varied are the forms presented by clouds 
that they seem, at first sight, to defy scientific classification. 
In 1802, however, Mr. Luke Howard, an eminent meteoro- 
logist, proposed, in an essay On the Modifications of Clouds, 
a system of nomenclature and classification, which has 
since been so commonly adopted that his terms are frequently 
used, even in popular descriptions of scenery. Reference 
to Plate III. will convey a better idea of the typical forms of 
clouds than could be got from any long technical descriptions.^ 

Delicate white fleecy clouds may often be seen floating 
in the upper regions of the atmosphere, where they are 
arranged in groups running in more or less parallel directions. 
Frequently a cloud of this class will present the appearance 
of hair, or feather, with its fibres curled, and hence it has 
received the name of cirrus} The cirrus clouds are always 
lofty, sometimes as much as ten miles above the surface of 
the earth ; and, being wafted along by currents in the upper 
regions of the atmosphere, they may often be seen to move in 

1 These figures are partly taken from Iiistmctions in the Use of 
fleteorological Instruments^ by R. H. Scott, M.A., F.R.S. 1875. 
^ Cirrus, a curl. 





M'^ 



kl^. 



,-MSg'iS^j^^W!:fM^!if^'^iWi'^Si^SW^i'^^---^^^^^ 



Vb: 







M 



%h 






wmk 



III.] RAIN AND DEW. 43 

a direction opposite to that of the wind which happens to be 
blowing over the surface. It is these clouds that are sup- 
posed to be made up of minute ice-particles (p. 41), since 
they produce, when they come between us and the sun or 
moon, those coloured circles which are known as halos. 

Very different from the cirrus is the well-known cumulus,^ 
a dense cloud which forms towering heaps of convex or 
concave form resting on a nearly horizontal base. Different 
again are the continuous sheets of cloud which are often to 
be seen extending widely in a horizontal direction, and are 
known consequently as the stratus.^ 

It often happens that the clouds under observation will 
not fall into any place in the foregoing classification. 
Instead of belonging definitely to any one class they may 
combine the characters of two or more groups, and in such 
cases expressive designations are framed by combining the 
preceding elementary terms. Thus, the beautiful effect of 
what is known popularly as the "mackerel sky" is due to 
numerous detached clouds of the composite forms termed 
cirro-cumulus. In like manner we may have a cirro-stratus 
and a cumulo-stratus, but such words sufficiently explain 
themselves. The dull rain-cloud, termed the nimbus,'^ is 
a composite form sometimes described as a cumulo-cii'ro- 
stratus. It is the dark grey cloud, or system of clouds, 
from which rain is actually falling. Before leaving the 
nomenclature of clouds, it may be useful to remark that 
the term scud is frequently applied to any loose detached 
clouds which drift rapidly before the wind. 

In meteorological reports, it is convenient to express 
approximately the proportion of sky which, at any given 
time, is covered by cloud. This is effected by using an 

^ Cumutus, a heap. * St7-atits or stratum, a bed. 

'^ Nimbus, a rain-cloud. 



44 



PHYSIOGRAPHY. [chap. 



arbitrary scale. Thus, a clear blue sky is represented by 
a cipher, while a sky completely obscured is designated 
by lo; the intermediate numbers from o to lo being used 
to express varying proportions of cloudiness. 

It has been shown that, when watery vapour condenses in 
the upper regions of the atmosphere, it gives rise to the 
formation of clouds. But, if the condensation occurs near 
to the surface of the earth, it produces those visible vapours 
which are known as mist or fog. Beyond the difference in 
the place of origin there is really little or no distinction 
to be drawn between a Tog and a cloud. A fog is a cloud 
resting on tlie earth ; a cloud is a fog floating high in 
the air. When in ascending a ' cloud-capped ' mountain, we 
enter the region on which the clouds rest, we find ourselves 
in a dense white fog. But after traversing this and reaching 
the clear summit, perhaps a thousand feet higher, the fog 
which we have left below once more appears under the 
guise of a mass of clouds. 

Whenever moist air near the surface of the earth has its 
temperature sufficiently reduced, the moisture may be con- 
densed as mist or fog. Thus fogs almost constantly hang 
over the banks off the coast of Newfoundland, where they 
are produced by the warm moist air of the Gulf Stream ' 
coming in contact with the cold air of the Labrador 
current. In like manner, icebergs are often attended by 
fogs, simply because the mass of ice cools the surrounding 
air, and thus precipitates its moisture. Mountains, again, 
are frequently enveloped in mist, since the warm air, on 
being driven up the slope of the hill, becomes chilled to 

1 It will be explained in Chapter XI. that the Gulf Stream is a body 
of warm water which flows from the Gulf of Mexico across the Atlantic 
Ocean ; the Labrador current is a body of cold water coming down 
from the north alon:' the coast of Lab.adur. 



in,] RAIN AND DEW. 4i 

the point at which its moisture is partially condensed. So 
too the position of a river is often marked by mist ; and 
this whether the water be colder or warmer than the 
overlying air : in the former case, the air is cooled down 
by contact with the water, and its moisture discharged ; in 
the latter case, the warm water yields more vapour than the 
air can retain at the given temperature. The British Isles, 
washed by warm water on their western shores, are peculiarly 
subject to fogs ; and, of all places, large towns seated on 
rivers are the most affected, since the artificial heat, coupled 
with the moisture of the air over the river, produces con- 
ditions favourable to the formation of fogs whenever the 
air becomes sufficiently cooled. The proverbial London 
fog owes its density and darkness to the smoke, or particles 
of carbonaceous matter, disseminated through the atmo- 
sphere and mingled with the partially condensed water. 

As long as water remains in the state of cloud, or fog, its 
particles are so minute that they hang suspended in the air, 
or rriount upwards on the slightest current. But, when these 
droplets run together, they produce drops too heavy for 
suspension in the atmosphere, and are then precipitated to 
the earth as rain. The rainfall, or amount of rain which 
falls in any given locality, is a most important element in 
determining its climate. 

What does a meteorologist mean when he says, in his 
technical language, that the annual rainfall in London is 
about 24 inches? By such a statement he means, simply, 
that if all the rain which falls on any level piece of ground 
in London during an average year could be collected — 
none being lost by drying up, none running off the soil, 
and none soaking into it — then, at the end of the year, it 
would form a layer covering that piece of ground to the 
uniform depth of two feet. The year's accumulation of rain 



46 PHYSIOGRAPHY. [chap. 

would thus form a vast mass of water. Remembering that 
an inch of rain represents about loo tons of water to the 
acre, it will be found that every acre of land in the metropolis 
receives during the year, when the year is neither very 
wet nor very drj', not less than 2,400 tons of rain. 

In passing across England from east to west, it is found 
that, as a rule, the rainfall increases. Thus, in the basin of 
the Thames, the fall varies, from about 24 inches in the 
eastern part, to between 30 and 40 inches among the western 
hills, in which the river takes its rise. Looking at the entire 
basin of the Thames, it may be said that the average rainfall 
is about 26 inches. Now the area of the basin, as already 
stated, comprises upwards of 6,000 square miles. Suppose 
then that we measured out a square space a mile in length 
on each side, and built upon this base a four-sided tower 
two and a half miles in height, which we completely filled 
with fresh-water ; this enormous column would represent the 
quantity of water which falls upon the surface of the 
Thames basin in the course of twelve months. And it 
should be borne in mind that every drop of this water has 
at some time, existed in the atmosphere as invisible vapour. 
In one sense, therefore, it maybe truly said that the Thames 
has its source in the air. 

Passing beyond the western limits of the Thames basin, 
it is found that the rainfall becomes heavier, until, in the 
western promontory of Cornwall, it exceeds 40 inches. The 
maximum, however, is attained among the mountains of 
Wales and Cumberland ; the wettest spot in England being 
near Seathwaite in Cumberland, where the average rainfall 
has been recorded as 165 inches. 

A general view of the rainfall throughout England and 
Wales is presented by the accompanying rain-map (Plate 
IV.), which is reduced from the map published in the 



III.] RAIN AND DEW. 47 

sixth report of the Rivers PoUution Commission, for which 
it was prepared by Mr. G. J. Symons. Such maps ave 
generally called Hyeto graphical'^ or Hyetological maps. In 
the one given here, the differences of rainfall are indicated 
by differences of tint, which are sufficiently explained by 
the accompanying index. 

In examining the distribution of rain, it will be found to 
be regulated partly by the physical features of the country, 
and partly by the character of the prevailing winds. In the 
neighbourhood of mountains, the rainfall is increased, since, 
as has already been pointed out, a mass of moist air, when 
forced up the side of a mountain, is chilled in the ascent, 
and its moisture consequently discharged. Among our 
western counties,, in the neighbourhood of hills, the rainfall 
rises to eighty, or even to a hundred, inches, and upwards ; 
while away from hills, though still in the west, it is only 
from thirty to forty-five inches. A table-land, or high plain 
surrounded by mountains, will generally receive but little 
rain, since the winds which reach it have been more or less 
drained of moisture in sweeping over the surrounding hills. 
For a like reason, but little rain is likely to fall on the lee 
side of a high hill, and many mountains, consequently, have 
a wet and a dry side ; the wet side being, of course, that 
towards which the predominant winds blow. As regards 
the influence of winds on rain, it is evident that, when air 
has blown over a large expanse of warm water, it must have 
become laden with moisture, which will be readily precipi- 
tated on exposure to refrigerating influences. Hence, as in 
Britain, so in the greater part of Europe, the southerly and 
westerly winds bring rain ; and most rain falls in the exposed 
westerly parts, such as the coasts of Portugal, Spain, France, 

^ From viTbs, hiietos, rain. 



48 PHYSIOGRAPHY. [chap. 

Britain and Norway. Tliere are certain conditions, how- 
ev^, under which rain is brought to our islands by easterly 
rather than by westerly winds. 

It is in those regions in which the sun's heat is intense, 
and powerful currents of highly-heated air, saturated with 
watery vapour, are raised into the atmosphere, that the 
heaviest rains occur. But the heavy tropical rains are usually 
confined to definite periods — the rainy season — and are not 
spread over the entire year, as in the temperate zone. 

The Khasi Hills (which lieabout too miles to the north-east 
of Calcutta) present the greatest rainfall in the world. Sir J. 
Hooker recorded upwards of 500 inches during a stay of nine 
months, and the total annual fall is about 524 inches. On the 
other hand, there are certain localities in which little or no rain 
falls : the chief of these rainless regions being Upper Egypt, 
the Sahara, the desert of Gobi in Central Asia, and the coast 
of Peru. As we recede from the hotter regions of the earth 
either to the north or south, the rainfall, as a rule, diminishes 
but the number of rainy days in the year increases ; so that, 
speaking roughly, it may be said that, where the rainy days 
are fewest, the amount of rain is gre-atest. 

In temperate regions, the number of rainy days varies 
greatly in different localities, and in different seasons. But 
it is difficult to know exactly what is meant by so vague 
a phrase as a " rainy day." To secure uniformity among 
observers, Mr. Symons has therefore proposed that meteoro- 
logists shall regard as a " rainy day," every day on which 
the rainfall is not less than one hundredth of an inch. 

It is matter of common observation that the rainfall 
varies not only in different localities, but in the same 
locality at different times. One year may be much wetter 
than another. A remarkable instance of this was presented 
by the exceptional rainfall of 1872, a fall which, in most 



Tlate iV. 




III.] 



RAIN AND DEW. 



49 

districts, was excessive, and in some was unprecedented. 
It is believed that no such fall has been recorded since 
observations were first instituted, now two centuries ago. 

It is curious to compare the 1872 fall with tl>at of the 
following year, which was remarkably dry. Thus, l\ir. 
Symons has recorded the rainfall in Camden Square, 
.London, as 33-86 inches in 1872, and only 22-67 inches in 
1873. But striking as the difference is here, it is yet more 
marked in other localities. At Barnsley, for example, tiie 
fall in 1872 was 42-28 inches, and in 1873 only 15-9 inches ; 
in other words, the rainfall in the dry year was but 38 per 
cent, of that of the preceding year. 

It may be useful, before quitting the subject of rain, to 
explain how the rainfall at any given 
station may be determined. Although 
the operation is extremely simple, 
numerous kinds of rain-gauge have 
been devised. The accompanying 
figure (Fig. 12) represents a simple 
form recommended in the Instnidions 
i?i the use of Meteorological Instruments 
(1875), compiled by Mr. R. H. Scott, 
director of the Meteorological Office. 
The instrument consists of little more 
than a circular metallic funnel for 
catching the rain, and a vessel for storing it. All the rain 
which falls upon the open mouth is collected, and, when 
collected, is exposed to but little loss by evaporation. The 
area of the collecting vessel varies in diff"erent forms of 
apparatus, the Meteorological Office employing a funnel 
eight inches in diameter. By means of the high cylinder 
around tile top of the funnel, snow may be collected ; but 
there are great difficulties in riaking accurate observations 




Fig. 12. — Rain-gauge. 



50 PHYSIOGRAPHY. [chap. 

on the fall of snow. It is notable that different amounts of 
rain will be collected by gauges placed at different heights 
in the same locality; a gauge at a low level always reading 
higher than one above. In all cases the instrument must, 
of course, be placed in a freely-exposed situation. Every 
morning, at nine o'clock, the rain collected during the 
preceding twenty-four hours is transferred from the col- 
lecting can to the graduated measure-glass, and its amount 
accurately recorded. 

Of the rain whirl, falls in any given district, such as the 
basin of the Thames, part is lost by evaporation and passes 
invisibly into the air ; at the same time another part soaks 
into the soil, and this also appears to be lost ; whilst part 
again flows off the surface of the ground to lower levels. 
Rain is thus disposed of in a threefold way, but the relative 
proportion between the three parts will vary considerably 
in different localities, and at different times in the same 
locality. It is dependent on climate and season, on the 
character of the soil, and on the physical features of the 
district. But whatever the proportion may be, the rain 
which is absorbed by the ground and that which flows otf 
the surface will contribute sooner or later to the formation 
of springs and streams.' And in this way the rains indirectly 
nourish the rivers, since we have already seen that rivers 
are mainly fed by springs and streams. The more rain which 

^ From what has been said respecting the origin of springs, it is 
obvious that, independently of the effects of evaporation, the quantity 
of water which reaches a river may be less than that which falls in the 
shape of rain upon its catchment basin ; inasmuch as some may go 
to feed the springs of other catchment basins. And, on the other 
hand, the quantity of water conveyed by a river may be indefinitely 
greater than that which falls upon its catchment basin, if the geological 
structure of the basin is such as to lead the rain from beyond its limits 
into the springs of the river. 



III.] RAIN AND DEW. 51 

falls upon the surface, tlie greater therefore will be the 
yield of the river. " Rivers," said Captain Maury, " are the 
rain-gauges of Nature." 

Amospheric moisture is frequently condensed in other 
forms than that of rain. If a glass of water recently 
drawn from a cold spring be brought into a warm room, 
it will be found that the outer surface of the glass gradually 
loses its brightness ; a dimness soon creeps over the surface 
that was previously clean and bright, and, before long, 
drops of water may be seen trickling down the sides of the 
vessel. It is true that certain kinds of glass, such as some 
of the old Venetian, constantly exude moisture, or " sweat," 
so that after the surface has been dried it soon becomes 
moist again — an effect probably due to an excess of soda 
in the glass. But the moisture which appears on ordinary 
glass, under the circumstances indicated above, is clearly due 
to an entirely different cause, since it is produced with 
equal readiness on glass of any chemical constitution, or 
indeed upon a vessel of polished metal. It is evident then 
that the moisture is not derived from the substance of the 
vessel itself; neither is it obtained by percolation through 
the walls of the vessel, for the metal has no sensible pores. 
The only remaining source of moisture is the surrounding 
medium, or the atmosphere. That medium always contains 
more or less watery vapour ready to be deposited upon any 
object when sufficiently chilled, and the necessary refrigera- 
tion is brought about by the cold water in the glass or the 
metal vessel. Moisture which is thus deposited upon any 
cold surface, without production of mist, is termed de7a. 

The proportion of watery vapour that can be held in the 
atmosphere depends principally on the temperature of the 
air ; the lower the temperature the less it retains. If charged 

E 2 



52 



PHYSIOGRAPHY. 



[chap. 



so highly with moisture that it can take up no more, the air 
is then said to be satiwated. When a body of moist air is 
cooled, the point of saturation is gradually reached ; and, 
when saturated, any further cooling causes a deposition of 
dew : hence the temperature at which this occurs is called 
the deiv-point. This point may be determined in a variety 
of ways, but it is interesting to note that some of the instru- 
ments used for this determination are based on the principle 
to which reference has just been made. Thus, the instru- 




FiG. 13. — Daniell's hygrometer 

ment designed by the late Prof. Daniell, and represented in 
Fig. 13, consists of a glass tube bent twice at right angles, 
and terminated at each extremity by a bulb : one of these 
bulbs, A, contains ether, while the other, B, is empty and 
inclosed in muslin. If a few drops of ether be poured 
?ipon this muslin, the ether-vapour within the tube is con- 
densed, and the liquid in A rapidly evaporates; but this 
evaporation is attended with reduction of temperature, and 



III.] RAIN AND DEW. 53 

consequently the bulb A is rapidly cooled. When the 
temperature of the surrounding air is sufficiently lowered, 
the dew-point is reached, and a film of moisture is then 
deposited on the outside of the bulb A. The temperature 
at which this takes place is indicated approximately by the 
thermometer inside the bulb, while the temperature of the 
■air outside is given by the thermometer placed on the 
upriglit stand. In another form of the instrument, devised 
by M. Regnault, the moisture is precipitated upon the sur- 
face of a small vessel of polished silver. It will be seen, 
however, that both instruments are but refined modifications 
of our familiar experiment with the tumbler of cold water. 

After sunset, on a clear night, the grass and other objects 
on the surface of the earth give off the heat which they 
have absorbed during the day, while the sun has been 
shining upon them, and their temperature is thus gradually 
lowered. Th© air in contact with these objects is also 
cooled ; and, as it gets cool, it grows less able to retain its 
moisture, until at length the dew-point is reached, when 
drops of liquid are deposited on the blades of grass. Some 
bodies throw out, or radiate, their heat much more freely 
than others, and dew is therefore precipitated copiously 
upon such good radiators. Thus it may sometimes be seen 
in a garden that every blade of grass is bedecked with 
sparkling dew-drops, while the neighbouring gravel path 
remains almost dry. The grass has parted with its heat, 
and consequently become chilled, more readily than the 
gravel has cooled ; and the dew is therefore distilled more 
abundantly upon the grass than upon the gravel. 

Whatever prevents the radiation or giving-off of heat 
from terrestrial bodies tends to hinder the formation of dew. 
A cloud, for example, acts in this way, since it reflects, or 
throws back upon the earth, the heat which would otherwise 



54 PHYSIOGRAPHY. [CH. iii. 

be projected into space. Dew is therefore most copious on 
a cloudless night. A calm atmosphere also promotes the 
formation of dew, for it is obvious that agitation by currents 
of air must be unfavourable to local refrigeration ; while it 
promotes the evaporation of any dew that may have been 
deposited. 

It was not until the beginning of the present century that 
so common a phenomenon as the formation of dew was 
thoroughly understood. Observations on the subject had 
indeed been made at a much earlier date, but it remained 
for Dr. W. C. Wells, an American settled in this country, to 
undertake a systematic inquiry into the conditions under 
which dew is deposited. After much patient investi- 
gation he published, in 1814, his famous essay entitled 
The Theory of Deiv, and the simple explanation which he 
set forth in this essay has been corroborated by subsequent 
investigators. 

Atmospheric moisture is precipitated not only as rain and 
dew — the particular forms which have been studied in tins 
chapter — but also occasionally as snow and hoar-frost ; 
the formation of these will foim the subject of the next 
chapter. 



CHAPTER IV. 

THE CRYSTALLISATION OF WATER : SNOW AND ICE. 

During the greater part of the year, in this country, 
the atmospheric moisture is condensed in a Hquid state, 
partly as rain and partly as dew. But, when the temperature 
of the air falls below the freezing-point, the water, unable 
any longer to retain this liquid condition, is reduced to a 
solid form ; and the moisture is consequently precipitated 
as snow instead of rain, and as hoar-frost instead of dew. 
It is of much importance to have some knowledge of the 
manner in which this great change in the physical condition 
of water is brought about. 

Daily observation shows that almost everything gets 
smaller as it is cooled. Reduction of temperature, as a 
rule, causes the particles of which a given body is composed 
to be brought more closely together, and the substance 
consequently shrinks in bulk. Suppose a quantity of air is 
confined in a vessel standing over water or quicksilver, and 
that it is found at a given temperature to occupy a certain 
bulk ; then, on lowering its temperature, the air will shrink in 
volume, or occupy less space, so that the water or quick- 
silver will tend to rise in the vessel, and thus fill the space 
which would otherwise be left by contraction of the air. 



56 PHYSIOGRAPHY. [CHAP. 

By careful observation it has been found that the shrinkage 
proceeds with great regularity as the air gets cooler, but 
there is no need to trouble ourselves, at present, with the 
law of contraction. 

Now watery vapour, such as that present in the atmo- 
sphere, is a body which may be said roughly to have a 
constitution similar to that of the air with which it is 
associated. But when this vapour is cooled, a limit is soon 
reached, beyond which any further cooling brings about 
the condensation of the vapour as liquid water. In fact, 
watery vapour, or steam, differs from fluids like air chiefly 
in the readiness with which it can thus be condensed or 
liquefied. 

Having in this way reduced the vapour to the condition 
of a liquid, it is important to observe the effect of still further 
lowering its temperature. As the water becomes cooled, the 
bulk of the liquid is diminished. With most liquids, this re- 
duction of bulk continues until their parts lose that freedom 
of motion upon one another which is characteristic of a 
liquid, and the mobile liquid passes into a compact rigid 
solid. The solid thus obtained by the congelation of water 
is termed ics. It is important however to note that water 
and a few other liquids, instead of continuing steadily to 
contract when cooled, reach a limit at which contraction 
stops and is succeeded by expansion, so that the solid 
water actually occupies a good deal more space than did 
the liquid from which it was derived. When a water- 
pipe bursts during a frost, or a jug of water cracks as 
the liquid freezes, we are practically taught that water, 
during the process of solidification, undergoes a large 
increase of bulk. 

By reason of this expansion, a piece of ice necessarily 
weighs much less than an equal bulk of water. If, for 



IV.] Sx\OW AND ICE. 57 

instance, a given bulk of water, measured at that tempera- 
ture at which its relative weight is greatest, be found to 
weigh 1,000 pounds, an equal bulk of ice will weigh only 
916 pounds. Hence ice floats readily on water, and floats 
with only about one-tenth of its volume above the surface. 
This may be seen by dropping a lump of ice into a tumbler 
of water, and observing how much is exposed, and how 
much buried in the water (Fig. 14.) Sea-water is denser, 
or heavier bulk for bulk, than fresh water ; and therefore a 
mass of ice floats higher in the ocean, about one-ninth of its 
volume being then exposed. Hence, in those huge masses 
of ice which are frequently seen floating in the sea and are 
known as icebe-rgs, the bulk of ice which 
is submerged is about eight times as 
great as that above water. But it must 
be remembered that the proportion of 
the submerged to the exposed part of 
the total height of t!ie berg will be 
aff"ected by the shape of the mass ; and 
probably, in many cases, the shape of Fig. 14.— ice in water, 
the berg is such as to cause it to sink 
to a very much smaller proportion of its total height beneath 
the surface than that represented m Fig. 14. 

It must not be supposed that the compact hard substance 
which is produced by the freezing or consolidation of 
water is a solid body without structure, like a piece of glass. 
Look at a bedroom window on a frosty morning, and you 
will probably find that some of the moisture present in the 
room has condensed upon the glass and frozen into solid 
ice; but you will see at once that this ice, instead of 
spreading itself uniformly over the surface, has shot out in 
definite directions, producing beautiful branching forms, not 
unlike the graceful fronds of a fern. The ice has, in fact, 




PHYSIOGRAPHY. 



[chap. 



assumed forms which are extremely definite in themselves, 
and are known as a-ystals. 

In the rocks of Snowdon, and of many other parts of 
Britain, there may be found a beautifully transparent sub- 
stance, of great hardness, which puts on very definite 
shapes. These shapes, as represented in Fig. 15, usually 
look like little six-sided towers, shooting in all directions 
from the rock, and terminated at one end, or sometimes 
at each end, by a short six-sided spire. The faces are as 
smooth and bright as though they had been just polished 




Fig. 15. — Rock crystal.' 

on the lapidary's wheel ; whilst the edges are as sharp and 
straight as though cut by a skilful workman. The ancients, 
who were familiar with these clear and colourless solid 
bodies as they occur in the granitic rocks of the Alps, 
supposed that they were composed of ice ; that they 



^ The lines on these ciyslals indicate shading, and not markings on 
the natural specimv?n. The prisms of rock-crystal are often marked by 
lines, but they run across the jirisms, not longitudinally in the direction 
of the shading in the figure. 



IV.] SNOW AND ICE. 59 

were, in fact, nothing but water which had been congealed 
by so intense a cold that it was impossible to thaw it. 
And the Greek word for ice {^pvaTuWoQ, krustallos), thus 
suggested our term crystal. Even at the present day many 
crystallised minerals are vulgarly called " congealed water." 
The substance which has just been noted as having given rise 
to the word " crystal " is known as rock-crystal, and must be 
familiar to most readers, since it is used by the jeweller for 
working into ornamental objects, and by the optician for the 
manufacture of those spectacle lenses which are said to be 
made of "pebbles." The term " crystal " is now applied 
to all symmetrical solid shapes assumed spontaneously by 
lifeless matter. 

Rock-crjstal is sometimes found in crystals of gigantic 
size ; at other times in excessively small specimens. This 
seems to show that the same species or kind of matter may 
assume forms unlimited in size. A few years ago some 
enormous specimens of dark-coloured rock-crystal were 
found in cavities of a rock above the Tiefen Glacier in 
Switzerland \ one crystal, christened the " Grandfather," 
weighing as much as 276 lbs., and another, called the 
" King," weigliing 255 lbs. Yet this same substance may 
be obtained in crystals so minute as to be seen only with 
aid of a microscope. Such great variation of size in the 
same kind of crystalline matter has no parallel among living 
bodies. It is true that certain animals and plants, placed 
under very favourable conditions, may increase beyond their 
average size, but this increase is confined within compara- 
'ively narrow limits. A crystal, however, has absolutely no 
limit to its growth ; it increases in size by addition of matter 
from the outside, and as long as new matter is thus presented 
to it, so long will it continue to enlarge. A small crystal oi 
alum, for example, suspended in a saturated solution of the 



6o PHYSIOGRAPHY. [chap. 

same salt, gradually grows larger by deposition of new alum 
in a solid form from the surrounding medium. This method 
of growth is therefore entirely different from that by which 
the growth of a living body is effected. It is as though a 
man could actually grow bigger by putting on coat after 
coat, instead of growing by the ordinary process of nutrition 
from within. 

Just as there is nothing distinctive about the size of an 
individual crystal, so there is nothing distinctive about the 
size of the several faces of the crystal. A six-sided spire or 
pyramid of rock-crystal may have one face very large, and 
the next face so small as to be little more than a mere line. 
Size of crystal and size of face thus count for nothing, but 
what does tell for something in studying crystals is the slope 
or inclination which one face has to another ; in other words, 
the angle made by two neighbouring faces. A set of faces 
symmetrically related, such as the six faces of the prism of 
rock-crystal, is called technically a form ; and the faces of 
any given form, however irregular in size and shape, are 
always inclined to one another at the same angle. 

Although it is not every substance that can assume these 
regular forms, yet by far the larger number of bodies, in- 
cluding water, are capable of crystallization. When water 
solidifies, by reduction of temperature, the particles group 
themselves in definite directions, and thus produce regularly- 
shaped solids, closely related to those of the rock-ciystal. 
In fact the forms of ice and the forms of rock-crystal are 
characterised by the same kind of symmetry ; a symmetry 
which is such that each crystal may be divided into six 
similar parts. The best examples of this hexagonal sym 
metry in solid water is furnished Dy crystals of siwiv. 

If the air during a snow-storm be still, each flake that 
falls will be found to exhibit a regular shape. A perfectly- 



IV. 1 SNOW AND ICE. . 6l 

formed snow-flake is, in fact, an exquisite little crystal ; but 
it commonly happens that a flake is made up of several of 
these crystals grouped together. Some idea of the beauty 
and variety of snow-crystals may be formed by reference to 
Fig. 1 6, which represents a few of the shapes observed by 




Fig. i6. — Snow-crj'stals 

Captain Scoresby in the Arctic Regions. More than a 
thousand different kinds have been described ; but various 
as these are, they are all characterized by the same kind of 
symmetry. Some of these snow-crystals are simply solid 
rods or flat scales, each with six sides ; others are six-sided 
pyramids, but the most common form is that of little 



62 rUYSIOGRAPHY. fcHAP. 

six-pointed stars variously modified. Each star has an icy 
centre as a nucleus, from which six little spicules or rods 
of ice are shot forth at regular angles ; and from the sides 
of these rays, secondary rays, or raylets, may be given off 
at the same angle, thus producing complex stars of great 
beauty, but, in spite of their complexity, always true to the 
hexagonal symmetry of the system to which ice belongs. 
Each part of the pattern is repeated round the centre six 
times, as is generally the case with the beautifully-sym- 
metrical shapes seen in a common kaleidoscope. 

Although ice does not ordinarily exhibit well-defined 
crystals, it is nevertheless built up of crystalline particles 
interlaced together. Prof Tyndall has shown us how to 
reveal this beautiful architecture, by submitting a block 
of ice to the action of a sunbeam, or even to a beam of 
electric light. Part of the heat enters the solid, and pro- 
duces internal liquefiiction, which proceeds with great 
regularity. Small shining points first appear in the ice, 
and around each of these points, as a centre, six rays 
shoot forth, producing figures such as those represented 
in Fig. 17.^ These beautiful forms, which commonly re- 
semble blossoms with six petals or floral leaves, are not 
solid crystals, like our crystals of snow, but are simply 
hollow spaces of regular shape filled with water ; they 
may indeed be called "negative" or "inverse" crystals, 
developed by the breaking-down or " decrystallisation " 
of the ice. The ice is in fact crystallitu, whilst the snow 
is crystallised. 

When there is much wind astir, the snow falls in 
shapeless masses, or even in small hardened pellets. If 
the snow-flakes become partially melted in their descent 
by faUing through a layer of warm moist air, they produce 

1 See Tyndall's Heal as a Mode of Motion, 5th ed. p. II. 



IV.] SNOW AND ICE. , 63 

wliat is called sleet. The largest snow-flakes fall when the 
temperature is near the freezing-point, and the smallest 
when the temperature is very low. It need hardly be 
said that snow is much lighter than rain ; it is usually 
estimated at about one-tenth the weight of an equal bulk 
of water, so that if a fall of snow lies on the ground to 



/ 



tL 













^r^'^ ^ .-^ 






c ;-^, ^.ii ^m^ 






Fig. '7.' — Ice-flowers. 

the depth of ten inches, it may be taken roughly to 
represent one inch of rain : it is obvious, however, that 
snow varies much in its state of compactness, and this 
metliod is consequently, in many cases, far from accurate. 
The loose texture of snow renders it an extremely 
bad conductor of heat, and a fall of snow thus acts like 
a mantle of fur thrown over the earth. The air entangled 



5^ PHYSIOGRAPHY. [chap. 

in the snow not only confers upon it this valuable propert)', 
but it also gives the snow its opaque white appearance, so 
different from the transparency of common ice. The light, 
instead of penetrating the snow, is thrown back from the 
ice-walls of each little air-cell or cavity, and thus becomes 
scattered, the snow losing its transparency ; just as the 
foam of the sea becomes opaque white, by the light being 
scattered from the particles of water into which a wave 
is broken. 

When snow falls upon a mountain in ^Yinter, it may lie 
there unmelted until the warmth of summer returns to 
thaw it. But, if the mountain be very high, the summer- 
heat may never be strong enough to melt all the ice on its 
top, and the top will therefore be enveloped in perpetual 
snow. A line drawn at the level above which the snow never 
melts is called the sncnv-line. On the north side of the 
Himalaya Mountains this line is 16,600 feet high ; that is 
to say, all the snow which falls below this height is melted 
in summer, but all above remains unmelted. In the Andes 
of Peru the limit of perpetual snow is about 15,500 
feet ; but in passmg northwards or southwards from these 
hot regions, we expect to find the snow-line descending ; in 
the Swiss Alps, for example, it comes down to about 8,500 
feet above the sea. Still farther north it reaches yet lower, 
and, in the Arctic regions, descends to the very sea-level; 
the winter's accumulation of snow is never completely 
melted by the summer-sun, and the snow consequently lies 
on the ground all the year round. 

Snow is not the only solid form in which atmospheric 
moisture is precipitated. Occasionally, during a storm, it 
takes the shape of hail, which consists of hard masses of 
ice varying in size from the smallest shot to pieces several 
inches in diameter. These hailstones are in some cases 



IV.] SNOW AND ICE. 65 

perfect spheres, as though the drops of rain had rapidly 
congealed while falling. When broken open, a hailstone 
occasionally exhibits crystals shooting out from the centre 
in all directions towards the surface ; but it is more usual 
to find a number of layers of ice, some clear and some 
opaque, coating a white snowy central mass, around which 
they appear to have been frozen in definite succession. As 
a rule, hail falls in summer rather than in winter, and in the 
day rather than in the night. The origin of hail is still 
obscure, but it is probably formed by an intensely cold 
current of air passing into a region of warm moist air, and re- 
ducing the temperature of the whole below the freezing point. 

There is yet another form of atmospheric precipitate 
that needs a passing notice. If the temperature after dew- 
fall should sink below the freezing-point, the moisture 
which would, under ordinary conditions be deposited as dew, 
takes a solid form, and is then known as /ioar-/rosf. Blades of 
grass, and other objects cooled by freely throwing off their 
heat into space, thus become coated with delicate icy crystals 
instead of dew. The hoar frost is, in fact, nothing but dew 
which has been frozen as it was formed. 

In one or other of the forms described in this and the 
preceding chapter, all atmospheric moisture must be pre- 
cipitated. It is not however always easy, nor is it by any 
means necessary, to distinguish between these several 
forms, and they are therefore practically massed together 
under the general head of " rainfall." If then it is said that 
the basin of the Thames is fed by a rainfall of twenty-six 
inches, what is meant is that the total quantity of atmospheric 
moisture precipitated within this area — adding together the 
rain and the snow, the hail and the dew — amounts in the 
course of an average year to a depth of six-and-twenty inches 
spread uniformly over the surface of the basin. 

F 



CHAPTER V. 



EVAPORATION. 



I'J whatever shape water may be thrown down upon the 
earth — whether as rain or dew, as snow or hail — it must, at 
one time, have existed in the state of invisible vapour 
diffused through the atmosphere, and not to be distinguished 
from the air itself. However dry the air may appear to be, 
it 3.1 ways contains more or less of this moisture. Though 
not recognized by the senses, its presence is readily revealed 
by the behaviour of certain substances which greedily absorb 
moisture, and are consequently said to be hygroscopic} Oil 
of vitriol, or sulphuric acid, for example, is one of these 
hygroscopic substances. If a bottle of this corrosive liquid 
be left without its stopper, it will be found that, after a few 
hours' exposure, the bulk and weight of the liquid have 
sensibly increased ; indeed a pound of oil of vitrol may in 
this way become two pounds in the course of a few days. 
This increase of weight is due to moisture absorbed from 
the surrounding air, and, after exposure, the acid is con- 
sequently found to be. weaker. When the air is damp, the 
increase of weight is rapid ; when dry, the increase is but 
slow. Yet the liquid can never be exposed, even to the 
driest air, witiiout absorbing some amount of moisture, how- 
Hyqrosopic, fromu-yoos ]u<gros, moist. 



CH. V.J EVAPORATION. . 67 

ever small. It is clear, therefore, that the atmosphere must 
always contam aqueous vapour. Nor is it necessary to seek 
far for its source. 

The damp tovi^el on which you have just wiped your wet 
hands does not stand long on the towel-horse before it 
becomes dry again ; the water left forgotten in the flower- 
vase a week ago has completely dried away. In such cases 
the water passes imperceptibly as vapour into the surround- 
ing air by a process termed evaporation. It is a quiet 
process, very different from the noisy production of vapour 
during ebidlitmi, or boiling ; yet the same in its ultimate 
result. The general process of converting a liquid into a 
vapour, by any means whatever, maybe called vaporisation ; 
and two modifications of this general process may be dis- 
tinguished — evaporation and ebullition. Whilst ebullition 
takes place only when the liquid undergoing vaporisation 
reaches a definite temperature, called its boiling poi?it, eva- 
poration is a permanent process going on at all times and 
in all places. Every piece of open water, from the narrow&st 
stream to the broadest sea, is constantly giving oft"" vapour in 
greater or less volume. More vapour will pass into the air 
on a hot than on a cold day ; yet, on the coldest day, the 
process of evaporation is simply slackened, not stopped. 
Even a piece of ice, exposed to air at the freezing-point, 
gradually diminishes in size, showing that vapour is given ofi' 
from the frozen surface. A fall of snow may evaporate, 
just as a shower of rain is dried up, but the process is 
immeasurably slower. It is therefore by no means difficult 
to account for the watery vapour in the atmosphere. And 
it should be remembered that, in addition to that which 
reaches the air by direct evaporation from river, lake, and 
ocean, there is a good deal of water thrown into the atmo- 
sphere as vapour by the agency of living beings, exhaled from 

F 2 



68 PHYSIOGRAPHY. [chap. 

the leaves of plants and from the lungs and skin of animals. 
Decay, and other chemical phenomena likewise contribute 
their quota to the moisture of the atmosphere. Evaporation, 
however, remains the principal source of watery vapour in 
the air. 

It need hardly be said that the rapidity of evaporation 
may be materially affected in a variety of ways. If you 
wish to dry a damp object quickly, you at once place it 
before the fire. Temperature, then, clearly affects the rate 
of evaporation ; the higher the temperature the quicker the 
process, other conditions remaining the same. Again, the 
amount of loss by evaporation is greatly affected by the 
hygrometric state of the air; in other words by the pro- 
portion of moisture already present in the atmosphere. If 
the air were perfectly dry, evaporation would be extremely 
rapid, and the loss correspondingly great ; if, on the other 
hand, the air were thoroughly saturated with moisture, water 
exposed to it would lose nothing. As a matter of fact we 
rarely, if ever, experience either one or the other of these 
extreme conditions ; but, between these extremes, there are 
any number of intermediate states. Every laundress knows 
that there are "good drying days" and bad ones. When 
there is but little moisture in the air the clothes dry quickly ; 
when there is much moisture, they dry but slowly. Let it 
not be supposed, however, that the proportion of moisture 
in the air is easily estimated by our sensations. True, we 
say that one day is dry, and another damp ; but, after all, it 
is not so much the absolute quantity of moisture in the air 
as its relative humidity that determines these sensations ; 
that is to say, it is the ratio of the vapour actually present 
to the amount which is capable of existing in the atmosphere 
at the given temperature. The higher the temperature the 
greater the quantity of water which may exist in the state of 



v.] EVAPORATION. ' 69 

vapour in the atmosphere, and consequently, on a hot day, 
the air may seem dry, notwithstanding it contains a large 
quantity of vapour, because much more might exist at that 
temperature. On the other hand, if the temperature be low, 
a small quantity of vapour may render the air damp, since it 
approaches nearer to the total quantity which can exist in the 
atmosphere at that temperature. Hence the paradox that, in 
summer, dry as the air may feel, it usually contains more mois- 
ture than in winter, when it is popularly said to be damper. 

Another condition affecting evaporation is the rapidity 
with which the atmosphere is renewed over the water 
exposed to it. On a windy day a wet pavement soon dries. 
The air in contact with the water soon receives as much, or 
nearly as much, watery vapour as is capable of existing at 
the temperature and thus further loss by evaporation is 
checked ; but, when the air is in motion, the portions which 
have become charged with vapour are rapidly removed and 
fresh ones brought into their place, which in turn become 
laden with vapour and are carried away to make room for 
others. It need hardly be said, too, that the rapidity of loss 
by evaporation depends on the extent of the exposed surface 
of liquid. Ink dries up quickly in a wide-mouthed inkstand, 
but the same quantity may be preserved much longer in a 
narrow bottle. In fact, the vapour is derived only from the 
exposed surface of the liquid, and herein lies one of the 
great differences between evaporation and ebullition : in the 
rapid process of boiling, bubbles of vapour are generated 
throughout the mass of liquid, while, in the slow process of 
evaporation, the vapour is derived from the surface only. 

Meteorologists occasionally measure the rapidity of evapo- 
ration by means of instruments called atmometers} It is 

' Atnioineier, from arr^xhs, afmos, vapour ; whence also atniospheret 
the spliere of vapour or air. 



7c 



PHYSIOGRAPHY. 



[chap. 



more useful however to determine the proportion of moisture 
in the atmosphere, and this determination is effected by 
instruments termed hygromeiers. The simplest but least 
trustworthy of such instruments depend for their action on 
the fact that organic structures readily absorb moisture and 
change their dimensions ; a hair, for instance, is longer when 
wet than when dry. Taking advantage of this fact De 
Saussure constructed the simple little 
instrument represented in Fig. i8. It 
consists of a human hair free from 
grease, stretched by a small weight, 
and furnished with an index, moving 
over a graduated arc. As the hair is 
affected by moisture the index moves 
over the scale, but its indications are 
not sufficiently exact to be of much 
scientific value. The instrument, 
though still used in certain parts of 
Europe, simply indicates the presence 
of moisture without accurately measur- 
ing its amount ; it is, in truth, a hygro- 
scojfe ra.th.er than a hygrometer.^ Ciuder 
even than this hair hygroscope is the 
well-known toy in the form of a little 
house with two doors, having the figure 
Fig. 1 8. —Hair hygrometer, of a man at One door and of a woman 
at the other. When the air is moist 
and rain may be expected, the man comes out ; when the 
air is dry, and the weather likely to be fine, the woman 

^ Instruments having names terminating in meter {filTpov, mdroii., 
measure) are generally more exact in their indications than those termi- 
nated in scope {crKOTrew, skopeo, to view). Thus a microscope enables us 
to see very minute obiects, whilst a micrometer enables us to measure 
them. 




v.] 



EVAPORATION. 



71 



makes her appearance. The movements of the figures 
depend on the effect of moisture upon pieces of catgut or 
"of twisted string. 

True hygrometers, or instruments for measuring humidity 
with considerable precision, have been constructed by 
Daniell, Regnault, and Mason, and are daily employed by 
meteorologists. Some of these instruments effect their 
purpose by indicating the dew-point directly, whilst others 
depend for their indications on the rapidity of evaporation. 
Daniell's hygrometer, a common in- 
strument of the former class, is repre- 
sented in Fig. 13, and described on p. 
52. The form of hygrometer now 
commonly used in this country is 
known as Mason's Dry-and-ivet bulb 
Thertnomefers, a name sufficiently dc 
scriptive of its construction. It consists, 
in fact, of two thermometers, placed 
side by side as represented in Fig. 19 ; 
one of the instruments has its bulb 
free, whilst the other is covered with 
muslin, which is connected, by means 
of a strand of cotton, with a small re- 
servoir of water : the thread constantly 
sucks up the liquid^ just as the wick of 
a candle draws up the melted wax or 
tallow, and the bulb is, in this way, 
constantly kept moist. Whenever a body passes from the 
state of liquid to that of vapour heat is absorbed : hence 
a httle water dropped upon the hand gives rise to the sen- 
sation of cold as it evaporates; a sprinkling of Eau de 
Cologne, or other liquid containing spirit of wine, produces 
greater cold, since it is more volatile than water and dries 




Fig. Tp.— Dry and wei 
bulb thermometers. 



72 PHYSIOGRAPHY. [chap. 

up much more rapidly ; a little ether, again, being still 
more volatile, reduces the temperature yet lower. The 
evaporation of the water from the wet bulb therefore 
lowers its temperature, and the more rapid the evaporation 
the greater will be this difference of temperature between 
the wet and the dry bulbs. If the air were saturated 
with moisture there could be no evaporation, and conse- 
quently the two thermometers would stand exactly alike. 
When, on the other hand, the air is very dry, evaporation 
becomes exceedingly rapid, and the temperature of the wet 
bulb consequently falls very low. From a comparison of 
the temperature shown by the two thermometers, the dew- 
point, the relative humidity of the atmosphere, and the 
quantity of vapour in a given volume of air, can be deter- 
mined by simple methods of calculation. Such an instrument 
as that just described is sometimes called a. Fsychi-ometer.^ 

From what has been advanced in this chapter, it is evident 
that more or less watery vapour is always to be found in the 
atmosphere ; its presence is constant, but its i)roportion 
variable. It niay perhaps be said that the air of England 
contains on an average something like i\ per cent, of 
aqueous vapour. This vapour is intimately associated with 
the other constituents of the atmosphere, all being gaseous 
bodies existing in a state of mechanical mixture. The 
composition of the atmosphere, however, is so important a 
subject that its full discussion must be reserved for the next 
chapter. 

When the temperature of the atmosphere is sufficiently 
reduced in any given locality, the watery vapour which it 
contains condenses as a liquid, while the other constituents 
retain their gaseous state. The liquid drops of water thus 
condensed as rain are said to be distilled. In fact, the 

' Psychrometer, from -^vy^pbs, psiickros, cold. 



v.] EVAPORATION. • 73 

process carried on in nature is precisely similar in principle 
to the artificial process of distillation. If it is required to dis- 
til a liquid, the liquid is evaporated in a boiler, and the 
vapour conducted to the condenser, where it becomes suffi- 
ciently cooled to be deposited in drops. The natural 
process is effected, not by boiling the water over a fire, but 
by the heat of the sun, which quietly steals vapour from 
every exposed piece of water, and the vapour thus raised 
into the atmosphere is ultimately condensed as drops of rain. 
In artificial distillation, any solid matter which happens to 
be dissolved in the original liquid will be left behind in the 
boiler, and the liquid consequently distils over in a state of 
purity, excepting so far as it may be contaminated by the 
presence of volatile matters. Just such a purification of 
water is effected by the natural process of distillation. The 
sea, which covers so large a proportion of the earth's surface, 
offers a vast exposure of salt water to the heat of the sun ; 
yet the salt is left entirely behind and nothing but pure water 
evaporated. Fresh water is thus being constantly distilled 
from the briny ocean. 

Thus, in seeking for the sources of the Thames, we are 
led from the springs of the earth to the rain of the heavens ] 
and from this to the watery vapour which forms part of the 
atmosphere ; and thence to the great caldron, the ocean, 
whence the heat of the sun distils that vapour. The great 
stream of fresh water which flows over Teddington Weir is 
fed, in large measure, by vapour which has been raised far 
away on the Atlantic. South and south-west winds sweep- 
ing across that ocean become highly charged with watery 
vapour ; and these warm moist winds, striking the 
Cotteswold Hills, deposit their freight of moisture in showers 
of rain, much of which reaches the Thames basin. Tliis 
water is ultimately carried out to sea by the flow of the 



74 PHYSIOGRAPHY. [CH. v. 

river, and mingles once more with its parent ocean, but 
only to be removed in due course by further evaporation. 
The waters of the earth thus move in a continued cycle, 
without beginning and without end. From rain to river, 
from river to sea, from sea to air, and back again from air 
to earth — such is the circuit in which every drop of water 
is compelled to circulate. The observer, who, looking down 
upon the Thames, watches the fresh water hurrying onward 
to the sea, must remember that the sea is not its resting- 
place, but that most of what he sees, perhaps all, will be 
distilled afresh and return to the earth in showers which 
may enter into the stream of Thames again ; or swell the 
affluents of some river on the other side of the globe ; or be 
secreted for untold ages in subterranean reservoirs. In the 
words of a wise man of old — "All the rivers run into the 
sea : yet the sea is not full ; unto the place from whence 
•the rivers come, thither they return again." 



CHAPTER VI . 



THE ATMOSPHERE, 



Every one is familiar with the common phenomenon of 
a piece of metal being eaten away by rust. A plate of 
polished iron or steel, for example, exposed to a moist 
atmosphere, soon loses its brilliancy, gradually becoming 
coated with a dull reddish-brown rust ; and this process of 
rusting, once set up, may go on until every particle of the 
original metal has disappeared. But let the same piece of 
bright metal be preserved in a vessel of pure water so as to 
avoid contact with air, and it may retain its lustre unimpaired 
for many years ; thus suggesting that the air must be directly 
or indirectly connected with the phenomenon of rusting. 
It is easy to show, indeed, that many metals rapidly rust or 
tarnish when exposed to even the driest air. Cut a piece of 
lead or of zinc, and observe the lustre of its fresh surface ; it 
is, in fact, almost as brilliant as a piece of polished silver, but 
this brilliancy is rapidly lost and the surface soon bedimmed 
on exposure to the atmosphere. On the other hand, there are 
many metals, such as gold, which never exhibit rust or 
tarnish, however long they may be exposed. Other metals, 
again, although they do not rust at ordinary temperatures, 
may be caused to rust more or less rapidly v;hen exposed to 



76 PHYSIOGRAPHY. [CHAP. 

the air at a high temperature. This is the case, for instance, 
with quicksilver. The rusting of this particular metal is 
worth closer study, since it was the means which led, about 
a century ago, to the discovery of the chemical composition 
of the atmosphere. 

Quicksilver, or mercury, as seen in the weather-glass, is 
as brilliant as solid burnished silver, and this bri-Uiancy is 
retained even after long exposure to air and moisture. But 
if the liquid metal be kept, for some time, at an elevated 
temperature in contact with air, small reddish scales slowly 
appear upon its surface, and ultimately the metal may be 
entirely converted into this substance. The red rust of 
mercury thus obtained is identical with a substance long 
known in pharmacy as " red precipitate," — a substance 
which is prepared commercially by other processes more 
convenient and rapid than that of heating mercury. 

It is especially notable that during the rusting of quick- 
silver, as indeed of all other metals, there is a very appreciable 
increase of weight in the substance operated on. A pound 
of metal produces considerably more than a pound of its 
rust. In point of fact, every loo lbs. of quicksilver will 
produce not less than io8 lbs. of red rust. This increase 
of weight shows that, during the operation of rusting, some- 
thing must be absorbed by the metal ; and as the mercury 
can be converted into rust when heated in contact with 
nothing but air, it is obvious that the additional matter 
must have been absorbed from the atmosphere. The nature 
of this absorbed matter may be determined by a simple 
experiment. 

Let a small quantity of red precipitate, or rust of mercury, 
be strongly heated in a tube of hard glass, represented at a, 
Fig. 20. If the tube be heated for a sufficient time the 
red powder may entirely disappear. But by making a bend 



Vi 



THE ATMOSPHERE. 



77 



in the tube, as at B, you may catch anything that distils 
over; and it will be found, at the end of the experiment, 
that this part of the tube contains metallic mercury. If loS 
grains of the red powder be heated in a, you may obtain in b 
loo grains of the liquid metal; in other words, you have 
expelled all the matter which has been absorbed froni the 
atmosphere during the process of rusting, and have regained 
the original weight of quicksilver. The matter which has 
been thus expelled from the powder by heat need not be 
lost ; for by attaching to the apparatus a tube c, which dips 




Fig. 20. — Decomposition of red oxide of mercury. 



beneath water in a vessel d, it will be found, on heating the 
powder in a, that bubbles of gas rise in the water ; and 
these bubbles may be conveniently collected in the bell-jar 
E. In this way you obtain a colourless and transparent 
gaseous body, not to be distinguished by the eye from 
ordinary air. Yet you have only to pmnge a lighted taper 
into it in order to see at once that you are dealing with 
something distinct from common air. The taper burns in 
it with unusual brilliancy ; and even if extinguished before 
entering the gas, so that only the merest point remains in 
a state of glow, this glowing point will be rekindled and 



78 PHYSIOGRAPHY. [chap. 

the taper burst again into full flame. The gas is, in fact, 
what is known to chemists as Oxygen. The red powder is 
a combination of this o.xygen with mercury, and is kno\vTi 
therefore as red oxide of mercury or mercuric oxide. When 
strongly heated, it is completely decomposed or spUt up 
into its constituents; every io8 grains of the red oxide 
yielding loo grains of metallic mercury and 8 grains of 
the gas oxygen. 

It was on the ist of August, 1774, that oxygen was 
originally discovered by Dr. Priestley. He obtained it 
from the red mercurial powder just as we have obtained it, 
excepting that he heated the powder by means of a large 
burning glass. Various other methods were soon discovered 
for obtaining the gas, and its properties were fully examined, 
especially by the Swedish chemist, Scheele, and the French 
chemist, Lavoisier. It was Lavoisier who gave to this 
curious kind of air or gas the name of Oxygen^ by which 
it is now universally known ; and it was he, too, who 
first showed, by the most conclusive experiments, what was 
really the composition of atmospheric air. His determina- 
tion of the constitution of the air was made in the year 1777. 
It is therefore only within the last century that chemists have 
become acquainted with the exact nature of so common 
a body as the air we breathe. 

Lavoisier took a weighed quantity of mercury and exposed 
it to strong heat in a vessel containing a confined volume of 
atmospheric air. In the course of twelve days the metal 
was completely calcined, or converted into the red rust or 
oxide. During this conversion the air diminished in bulk 
while the quicksilver increased in weight ; in fact, the 

' Oxygin, from 6^vs, oxtts, acid ; y^vvdoi, geniiao, to produce ; a 
name based on the supposition that substances burnt in oxygen always 
produce acid compounds. 



vr.] THE ATMOSPHERE. ; 79 

mercury had taken oxygen from the air, and combined 
with it to form the red oxide, from which, by stronger 
heating, the oxygen gas could easily be recovered in a 
St. te of purity. It remained, however, to inquire what 
was the character of the air left in the vessel which had 
been thus robbed of its oxygen. On plunging a taper 
into the residual air it was at once extinguished ; and on 
introducing a living animal into the air, the creature was 
suffocated. The latter property suggested to Lavoisier the 
propriety of giving to this foul kind of air the name of 
Azote ;^ — a name vi^hich it still retains in France, but which 
has been superseded elsewhere by the term Nitrogen?' 

On accurately examining a given measure of atmospheric 
air, it was found that it contained about one-fifth its bulk of 
the gas oxygen and four-fifths of nitrogen. To speak with 
more accuracy, 100 volumes of pure air contain 20"8 vols. 
of oxygen and 79*2 vols, of nitrogen. If instead of a 
given volume, or measure, a given weight of air is examined, 
it will be found that 100 parts by weight — whether grains, 
ounces, or pounds — contain 23 of these parts of oxygen 
and 77 of nitrogen. 

Before proceeding to examine more closely into the 
composition of atmospheric air it may be well, to note 
the characters of the two constituents into which it has 
just been seen that air may be resolved. In most of the 
chemical phenomena in which atmospheric air takes part 
it is the oxygen which is the active agent. It has been 
shown that a glowing taper bursts into flame when plunged 
into oxygen. In like manner sulphur, phosphorus, charcoal, 
even iron-wire, will burn with great vigour in this gas ; 

^ Azote, from the Greek privative d, and ^0177, zoe, life. 
* Nitrogen, from nit7-e, in consequence of nitrogen being a constituent 
of the salt called nitre or saltpetre. 



8o PHYSIOGRAPHY. [chap. 

the combustible substances in all cases combining with the 
oxygen to form oxides. Some of these oxides are solid 
substances, whilst others are gaseous. Every act of com- 
bustion in air depends on the presence of oxygen. When 
a piece of magnesium wire burns with its dazzling splendour, 
the metal combines with the oxygen of the air to form 
oxide of magnesium or magfiesia, which, after the combus- 
thus is left behind as a light white solid substance. When 
a piece of charcoal burns in air, the solid disappears, with 
exception of a little ash ; the charcoal has, in fact, combined 
with oxygen to form an oxide which is an invisible gas known 
as carbon dioxide or more commonly as carbonic acid. All 
our ordinary combustibles — such as coal, wood, oil, tallow, 
and wax — contain a large proportion of carbon ; and, con- 
sequently this gas is produced in considerable volume 
during their combustion. In like manner, the respiration of 
animals depends upon the presence of oxygen in the 
medium by which they are surrounded, whether air or 

water. A kind of slow combustion goes on in the body ; 
and the oxygen, taken into the system through either lungs 
or gills, is, in part, consumed in the formation of carbonic 
acid gas, which is expelled through the same organs. Oxygen 
is therefore as needful to support animal life as to support 
flame, and hence it was at one time known as " vital air." 
After death, again, the matter which was once living is 
subject to a process of oxidation or slow combustion, by 
which it is converted, for the most part, into compounds 
which contain a larger proportion of oxygen. Oxygen is 
therefore essential to the maintenance of combustion, respi- 
ration, decay, and many other natural and artificial processes 
in daily operation. In pure oxygen, all these actions would 
be carried on with undue energy, and the nitrogen of the air 
plays an important part in tempering the activity of the oxygen 



^i.] THE ATMOSPHERE. • 8i 

with which it is associated. This nitrogen is remarkable 
for its inertness ; it extinguishes flame and it does not 
support life ; yet it kills, not by being absolutely poisonous, 
but simply by excluding oxygen. A fresh supply of air 
is therefore constantly required by a living animal, not 
because the nitrogen is deadly, but because the needful 
oxygen is absent. 

But although nitrogen is not a dangerous gas, there are 
other gaseous bodies always present in the atmosphere which 
in a pure state are active poisons. Let a saucer of clear 
lime-water be exposed to the air, and in a few hours the 
surface of the liquid will be covered with a thin pellicle of 
whitish matter ; this is produced by something absorbed 
from the atmosphere, yet neither oxygen nor nitrogen 
produces the effect. It is due, indeed, to the presence of 
the gaseous substance to which reference has already been 
made under the name of carbonic acid gas ; this gas, acting 
on the lime-water, forms a solid carbonate of calcium, or, as 
it is more commonly termed, carbonate of lime ; and it is 
this white solid substance which forms the thin skin on the 
surface of the water. Carbonic acid gas, which is thus 
proved to exist in the atmosphere, is a compound of two 
distinct substances — carbon and oxygen. The oxygen has 
been already described : the carbon is a solid body abun- 
dantly distributed through nature, though rarely occurring in 
a state of purity. In its purest native form, it crystallises as 
the diamond ; in a less pure condition it constitutes graphite 
or " black-lead j " and, in chemical combination with other 
substances, it forms a large proportion of coal and of all 
other ordinary forms of fuel. It enters largely, too, into the 
constitution of all living matter, whether animal or vegetable ; 
and it is left (mixed with a greater or less proportion of other 
substances) when these substances are charred or imperfectly 





82 PHYSIOGRAPHY. [cHAf. 

burnt, as in coke, wood-charcoal, animal-charcoal, &c. 
During all the processes of combustion, respiration, and 
decay, this carbon combines with the oxygen of the air to 
form carbonic acid, and hence this gas is constantly being 
poured into the atmosphere. Breathe through a straw into 
a glass of clear lime-water, and you will see that the liquid 
becomes milky as the carbonic acid gas expired or breathed 
out from your lungs bubbles through the previously limpid 
liquid. If you then pour a little vinegar into the cloudy 
liquid, the milkiness immediately clears up, because the acid 
dissolves the solid white carbonate of lime which had been 
formed by your breath. Carbonic acid gas is set free by the 
action of the vinegar ; and, if there is enough of the solid 
carbonate in the lime-water, you may actually see the gas 
escaping in little bubbles. This bubbling, or effervescence, 
is likewise produced when vinegar, or almost any other acid, 
is poured upon an egg-shell or an oyster-shell, upon a piece 
of chalk or limestone or marble. All these substances con- 
sist, in truth, of carbonate of lime, and are decomposed by 
the acid with evolution of carbonic acid gas. If Cleopatra 
ever dissolved the pearl, as the story tells, or Hannibal 
softened the rocks of the Alps with vinegar, a chemical 
decomposition was effected exactly like that just described. 
In consequence of the gas being thus, as it were, bound in 
various solid substances, its discoverer. Dr. Black, of Edin- 
burgh, bestowed upon it the name oi fixed air. A taper 
plunged into this air is at once extinguished, and an animal 
is suffocated. Hence the great necessity of duly renewing 
the air in dwelling rooms. And it is obvious that, the 
greater the number of people in the room, and the greater 
the number of gas-burners, lamps, or candles alight, the 
more need is there of efficient ventilation. 

As carbonic acid gas is being constantly produced by 



Vl.] THE ATMOSPHERE. 83 

such processes as combustion and respiration, it is clear that 
the proportion of this gas in the atmosphere must vary 
locahy, being, for example, higher in a crowded space than 
in the open country. The average proportion of carbonic 
acid in the air may be estimated at from '03 to "04 per 
cent, by volume ; thus ten thousand gallons of air will 
contain between three and four gallons of carbonic acid. 
Dr. Angus Smith has published, in his work on " Air and 
Rain," a large number of analyses of air from various 
localities, with the view of determining the variation in 
the proportion of carbonic acid ; and, from his analyses, 
the following examples are selected : — 

Percentage of Carbonic Acid in Air. 

On the Thames at London, mean, '0343 

In the streets of London, ,, ....... '0380 

From the top of Ben Nevis '0327 

From the Queen's ward, St. Thomas's Hospital . . "0400 
From the Haymarket Theatre, dress circle, at 

11.30 P.M '0757 

From Chancery Court, 7 feet from ground . . . '1930 

From Underground Railway, mean, "1452 

From workings in mines, average of 339 samples . 7850 

Largest amount in a Cornish mine 2*5000 

These figures express percentages, but it is obvious thai 
they may be read as whole numbers per million. For 
example, instead of saying that air from the streets of 
London contains on an average "0380 per cent., it may 
be said that a million gallons of the air contain 380 
gallons of carbonic acid ; that a million gallons of air from 
the Thames contain 343 gallons of the gas ; and so on. 

Since the atmosphere is constantly receiving vast volumes 
of carbonic acid from various sources, it might not un- 

G 2 



H PHYSIOGRAPHY. [chap. 

naturally be assumed that this gas would unduly accumulate, 
and at length vitiate the entire bulk of the atmosphere. 
Such accumulation is, however, prevented by the action of 
living plants. To show tliat so small a proportion of car- 
bonic acid in the atmosphere as 0-035 V^^ cent, is sufficient 
to supply the vegetable world with its carbon, it is simply 
necessary to calculate the weight of this gas in the atmo- 
sphere resting on a square mile of land. The weight of air 
on this area is about 59,012,997,120 lbs., (or 26,345,088 
tons), and the carbonic acid which it contains weighs not 
less than 13,800 tons. The weight of carbon in this car- 
bonic acid is about 3,700 tons. The carbonic acid, so in- 
jurious to the animal, is the source whence ordinary plants 
derive the whole of the carbon in their structure. Wood, 
for example, contains about half its weight of carbon ; yet 
every particle of carbon in a forest of trees has been de- 
rived from the gaseous carbonic acid invisibly distributed 
through the surrounding atmosphere.^ 

Before leaving the subject of carbonic acid, it should be 
remarked that this gas is one of great densitj'', being in fact 
about half as heavy again as an equal bulk of atmospheric 
air. It might, therefore, not unfairly be assumed that the 
carbonic acid in the atmosphere would tend to settle down 
in a stratum near the ground. If we shake up a mixture of 
liquids of different densities — say mercury, water, and oil — 
the liquids soon settle down, after agitation, in the order of 
their relative weights ; the heavy quicksilver sinking to the 
bottom, and the light oil floating on the top of the water. 
Such a separation does not however take place v/hen g-ases 
of different densities are mixed. The foUowins table shows 
the densities, or specific gravities, of the three gases which 
compose the atmosphere : — 

^ This subject will be further discussed in Chapter XIV 



vi.] THE ATMOSPHERE. - 85 

Nitrogen .... 0-9713 

Oxygen i'io56 

Carbonic-acid gas . . i'5^o3 

The term specific gravity is used to denote the weights of 
equal bulks of different kinds or species of matter, compared 
with some known standard. Air is the standard used in the 
comparison just made, and it is seen from the figures that 
if a given bulk of atmospheric air weighs 100 pounds, then 
the same bulk of nitrogen weighs 97 pounds ; the same 
volume of oxygen no pounds, and of carbonic acid 152 
pounds. Hence it might be assumed that the atmosphere 
would consist of three strata or layers (like the mixture of 
quicksilver, water, and oil), with the nitrogen as the top 
layer, and the carbonic acid at the bottom. As a matter of 
fact, however, this is not the case. All gases tend to inter- 
mingle with each other, so that when different gases are 
mixed they soon i^roduce a uniform mixture, in spite of 
differences in their relative weights ; in fact, the particles of 
the heavy gas rise and the particles of the light gas fall, 
until they are completely diffused through each other. In 
consequence of this property, the composition of the 
atmosphere is kept practically uniform, although local 
variations, within narrow limits, may be detected. 

In addition to the oxygen, nitrogen, and carbonic acid, the 
atmosphere always contains other constituents, but only in 
subordinate and variable proportions. The gas called am- 
monia, well known as the pungent gas which escapes from 
" spirit of hartshorn," is constantly present in the air, being 
indeed evolved from decomposing animal and vegetable 
matter. Yet the proportion of am.nonia is always excessively 
small ; for example, twenty grains have been obtained from a 
million cubic feet (or 536 million grains) of air. This ammonia 



86 PHYSJOGRAPHY. [chap. 

is a compound of nitrogen wiiii a gas called hydrogen, which 
will be described in the next chapter ; it is necessary however 
to refer briefly, in this place, to the composition of ammonia, 
since this gas, though present in only such minute proportion, 
directly or indirectly furnishes to plants a large part of 
their nitrogen, just as the carbonic acid gas supplies thera 
with their carbon. Traces of fiitric add, the substance 
known commonly as agua/ortis,SirQ occasionally found in the 
atmosphere, especially after thunderstorms; this nitric acid 
i"eadily combines with the amnionia to form nitrate of am- 
monia, the presence of which may frequendy be detected in 
rain water. Sulphuretted hydrogen, an offensive gas given 
off during the putrefaction of animal and vegetable matter, 
may also be often found in the air ; and a few other gases 
are sometimes present, especially in air taken from the 
neighbourhood of large towns. Nor should mention be 
omitted of the organic germs, which constantly float in 
the atmosphere, but of which it is beyond our present pur- 
pose to speak. As to the wateiy vapour, which is ever 
present in the air, it is unnecessary to say anything here, 
since the subject was fully discussed in the last Chapter. 

This watery vapour differs from the other constituents of 
the atmosphere principally in the ease with which it may be 
condensed or liquefied. Hence it is generally called a 
vapour rather than a gas. Yet there is^jeally but little 
distinction between the two classes of bodies ; a vapour 
being nothing more than an easily-condensible gas. Steam, 
for example, is liquefied by a comparatively slight reduction 
of temperature, while carbonic acid and a number of other 
gases require a great reduction of temperature or a great 
•pressure, or even a combination of cold and pressure, in 
order to assume the liquid form; and, until quite recently, 
several of the gases had resisted all attempts to liquefy 



vi.l THE ATMOSPHERE. . '87 

them, and were therefore termed permafient gases. In 
the closing months ot 1877, however, M. M. Pictet and 
Cailletet succeeded in bringing even the most refractory 
gases, such as oxygen, hydrogen, and nitrogen, into the 
h'quid state. 

When a Hquid is evaporated, or converted into gas or 
vapour, it undergoes a great increase of bulk, but its weight 
remains unaffected. A pound of water, for example, pro- 
duces neither more nor less than a pound of steam. It is 
clear, therefore, that gases and vapours, although generally 
invisible, must possess weight ; but this weight is necenisarily 
small compared with that of the same bulk of matter in 
the liquid or solid state. Atmospheric air is, in fact, about 
800 times lighter than an equal bulk of water, and as much 
as 11,000 times lighter than an equal volume of quicksilver. 
Yet the weight of air, small as it seems, amounts to some- 
thing considerable when we are dealing with a large bulk, 
or even with such a quantity as is contained in an ordinary 
dwelling-room. It is found by actual weighing that 100 
cubic inches of air, under ordinary conditions, weigh about 
31 grains ; in other words, it requires 13 cubic feet of air to 
weigh a pound avoirdupois. Suppose then that we have a 
room measuring 10 feet long, 10 feet wide, and 10 feet high: 
this will contain x,ooo cubic feet of air, and the weight of 
this air will be about 77 pounds. But the room just taken 
is a very small one, and if the calculation be extended 
to a large public building it will be found that the air 
which it contains weighs more than is commonly imagined. 
Thus, Westminster Kail has a length of 290 feet, a 
width of 68 feet, and a height of no feet; its contents 
must therefore be 2,169,200 cubic feet, and the weight of 
the air in this hall reaches the enormous amount of nearly 
7 5 tOTlS I 



88 PHYSIOGRAPHY. [chap. 

Since air possesses weight, it necessarily presses upon any 
object exposed to its influence. The atmosphere forms an 
ocean of air bathing the entire earth ; and, on the floor of 
this ocean, man, in common with all terrestrial beings, has 
his dwelling. Everything around us on the earth's surface 
must therefore bear the pressure of the air above, just as 
anything on the bed of the ocean is pressed upon by the 
superincumbent water. The depth, or rather the height, oi 
this aerial sea has never been determined, but there are 
reasons for believing that the atmosphere extends to at 
least 50 miles upwards from the earth's surface. Hence 
it is clear that all terrestrial objects must be subjected to 
enormous pressure. The roof of a house, for example, has 
to bear the pressure of a column of air resting upon its 
surface and extending upwards to the limit of the atmo- 
sphere. Nosv, it is found that our atmosphere exerts 
a pressure of nearly 15 lbs. (1473 lbs.) on every exposed 
square inch of surf^ice. The roof is consequently pressed 
upon by a weight of many tons. Yet the most delicate 
structure may be freely exposed to the atmosphere without 
the slightest danger of being crushed. This arises from 
the fiict ihd.i Jliiids^ transmit pressure in a manner entirely 
different from that m which it is transmitted by solid 
bodies. A solid presses downwards only, but a fluid 
presses equally in all directions, upwards as well as down- 
wards. The air in a room, for instance, presses on the 
ceiling not less than on the floor ; and on each of the 
walls not less than on the ceiling. Under ordinary con- 
ditions, therefore, the atmosphere has no power to crush, 
because its pressure downwards is exactly neutralised by 

1 Fluid, from fluo, I flow ; a term embracing both liquids and gasei 
or vapours, since th»i particles of botli classes oC bodies flow freely o^cr 
each ctl'.sr. 



Vl.J THE ATMOSPHERE. 8? 

its pressure upwards. Extend your hand, and you feel no 
pressure, though it is certain that every square inch of its 
surface bears a pressure of nearly 15 lbs., and the entire 
hand must therefore sustain a very large total pressure ; 
but the weight upon the upper surface is counteracted by 
the upward pressure of the air on the under surface, the 
two equal and opposite pressures neutrahsing each other. 
Nor is there any tendency for the hand to be crushed 
between these opposing pressures, for the air and other 
fluids in the vessels and various tissues of the body press 
equally in all directions, so that any pressure from without 
is perfectly counterbalanced by an equal pressure from 
within. The thinnest soap-bubble sails safely through the 
air, though its outer surface must sustain a pressure of 
many pounds ; for the air within the bubble presses just as 
forcibly against the inner wall, and thus resists the external 
atmospheric pressure, and effectually prevents collapse. In 
the common toy known as "Jack in the Box" a spring 
inside the figure presses upwards against the lid when 
tightly shut down ; and in like manner the walls of a 
closed vessel containing air are pressed outwards by the 
elastic force of the confined air. If the air be removed 
from the interior of a closed vessel, so as to leave a 
space altogether empty or vacuous, the pressure of the 
external atmosphere becomes at once evident, since it 
is no longer counterbalanced by any force from within ; 
a thin glass vessel, for example, may easily be shattered 
by sucking the air from its interior. 

It is easy to measure the amount of this atmospheric 
pressure by a simple experiment, first made in 1643, by an 
Italian philosopher named Torricelli. Take a glass tube, 
rather more than 30 inches in length, closed at one end 
and open at the other; fill this tube with quicksilver, and 



90 



PHYSIOGRAPHY. 



[chap. 



closing the open end with the thumb, as shown in the 
right-hand figure of Fig. 21, invert it in a basin of mercury 
so that the open end may dip beneath the Hquid ; it will 

then be found that the mer- 
cury falls for a short distance 
in the tube, but that a column 
about 30 inches in length re 
mains suspended, as shown 
in the left-hand figure. Tor- 
ricelli argued that this column 
must be supported by the 
pressure of the external at- 
mosphere on the surface of 
the mercury ; the downward 
pressure of the column of mer- 
cury being exactly balanced 
by the upward pressure of 
the atmosphere transmitted 
through the quicksilver. In- 
deed, if we admit air by 
making a hole in the top 
of the tube, the column im,- 
mediately falls, since it is 
then pressed down by the 
atmosphere above • but when 
the tube is closed there is no 
atmospheric pressure on the 
top within the tube, for the 
space above the column of mercury is completely empty, or 
rather contains only mercurial vapour, whence it is called 
the Torricellian vacuum. Since the column of mercury in- 
side is balanced by the atmosphere without, it follows that if 
we know the weight of the mercury we know also the weight 




Fig. 21. — Toriicelli's experiineiu. 



VI.] THE ATMOSPHERE. 91 

of a column of air standing on a similar base and extending 
upwards to the extreme limit of the atmosphere. Now a 
column of mercury 30 inches long, in a tube of one 
square inch in sectional area, weighs about 15 lbs.; hence 
it is found, as before stated, that the weight or pressure of 
the atmosphere is about 15 lbs. on every square inch. 

If instead of using a dense liquid, like mercury, the ex- 
perimentalist took a lighter one, such as water, he would 
naturally expect that the column required to balance the 
weight of the external atmosphere would be proportionally 
longer. As a matter of fact it is found that, when water is 
used, the suspended column is something like 33 feet in 
length : in other words, as water is about 13^ times lighter 
than mercury, bulk for bulk, the column of water will be 
about 13I times longer than the mercurial column. It was 
indeed, by observing a body of water raised in the suction, 
pipe of a pump at Florence, that Torricelli was led to his 
experiment with quicksilver. In working a common pump, 
air is sucked out of the tube communicating with the source 
of water below, and the pressure of the atmosphere then 
forces the water up the pipe in order to supply the place 
of the air which has been removed. But, when the pipe 
reaches a length of more than about 30 feet, the column of 
water which it contains balances the atmospheric pressure, 
and consequently if the tube be longer than this no more 
water rises, and the pump ceases to act. In seeking to 
find out why water cannot rise higher, Torricelli was led to 
make the experiment to which reference has been made, 
and to construct the instrument which is represented on the 
left-hand side of Fig. 21. It is called a Barometer?- 

1 Barovuter, from ^dpos, baros, weight, and fxkrpov, metron, measure ; 
an instrument for measuring the weight of the atmosphere. Thermo- 
meter, from Bep/xoi, thermos, hot ; an instrument for measuring tem- 
perature. 



92 PHYSIOGRAPHY. [chap. 

Various forms have been given to the barometer, but 
with the exception of the aneroid'^ — an entirely distinct 
instrument — they all depend on the same principle, namely, 
that of causing a column of liquid to be balanced against 
the weight of the atmosphere. Almost any liquid may be 
employed, but as a matter of convenience, mercury is the 
only substance in common use.^ 

As the pressure of the atmosphere in any given locality 
varies from day to day, and even from hour to hour, the 
height of the mercurial column is subject to corresponding 
fluctuation. The great use of the barometer, in fact, is to 
indicate these changes of atmospheric pressure — changes 
which are of vast importance to the meteorologist, since they 
are related to general changes in the weather. Not that the 
barometer forms a " weather-glass " as popularly understood : 
it does not indicate absolutely the character of the forth- 
coming weather, and the indications given on the dial of 
common instruments are scarcely of any scientific value. 
But still the changes in atmospheric pressure point to 
changes in the winds ; and these are the prime movers in 
effecting changes in our weather. Hence, the readings of 
the barometer form the chief element in the weather-charts 
and reports issued of late by most of the London daily 
papers, and as these are forced every morning upon the 

^ Aneroid, from the privative d, and vriplis, neros, moist ; an instrument 
in which the pressure of the atmosphere acts upon a thin elastic metal 
case, the movements of vifhich are transmitted to a dial. 

* Water-barometers have occasionally been constructed, but their 
great length renders them unwieldy, and they are also open to other 
objections. One of these instruments may be seen in the Museum of 
Practical Geology in Jermyn Street. Glycerine has also been used by 
Mr. J. B. Jordan, as may be seen in an instrument erected by him at 
South Kensington. But for all ordinary purposes mercury is invariably 
used. 



VI.] 



THE ATMOSPHERE. 



93 



reader's attention it may be worth while to explain the kind 
of information which they give, and how they are to be 
interpreted. 

Fig. 2 2 is a reproduction of the weather-chart given in the 




Fig. 22. — Times weather chart. 



Times of to day (March 31, 1877), and it shows the state 
of the weather at 6 p.m. yesterday. Tlie most striking 
feature in the chart is the series of dotted curved lines, 



94 PHYSIOGRAPHY. [cHAP. 

which are called isobars?- An isobar is simply a line con- 
necting all those places which have, at a given time, the 
same barometric pressure. Thus the first isobaric line, 
reckoning from the bottom of the chart, passes through the 
south of Ireland and the south-west of England, and then 
sweeps in a bold curve through France into the Bay of 
Biscay, At all points along this course the barometer stood 
at 3o'i inches, as indicated by the figures at each extremity 
of the curve. The next isobar, passing across the north of 
Ireland and England, is marked 30 inches, so that between 
the two lines there is a difference of pressure equal to that 
of one-tenth of an inch of mercury. Another isobar 
stretches across Scotland, and indicates a pressure of 29-9 
inches ; and the last curve on the chart is drawn through 
the north of Scotland, where the mercury stood at 29*8 
inches. The chart therefore shows at a glance the distri- 
bution of atmospheric pressure over the area represented, 
and from this we can learn a good deal about the character 
of the winds. Between two successive isobars there is a 
difference of pressure represented by one-tenth of an inch 
of mercury, and the distance between these two lines gives 
us the gradient. This term is familiar enough in engineer- 
ing ; if a railroad, for example, rises one foot for every 
100 feet of distance, the line is said to have a "gradient " of 
I in 100. The gradient is therefore the engineer's expres- 
sion for the slope of the ground ; in like manner it is the 
meteorologist's expression for what has been called the slope 
of the atmosphere. Only, in talking about meteorological 
gradients, it must be borne in mind that the vertical 
scale is measured in hundredths of an inch of barometric 
pressure, while the horizontal scale is measured in miles of 
distance, the unit being one degree, or 60 nautical miles. 
' Isobar, from ifo-os, isos, equal ; and ^dpos, baros, weight. 



VI.] THE ATMOSPHERE. 95 

Hence a gradient of 4 means that over a distance of 60 
nautical miles the barometer rises -—0 oi" A" ^^ ^^^ ^'^^'^- ^^ 
the isobars run close together it shows that the gradient is 
high, and therefore the winds will be strong; if they are 
wide apart the gradient is low, and the winds are light. 
Thus, in Fig. 22, the isobars indicate only light winds. 

Although much may be learnt about winds by studying 
the isobaric lines, it must not be supposed that the air blows 
directly from regions of high pressure to those of low 
pressure. Prof. Buys Ballot of Utrecht has however laid 
down a law which gives the exact relation of winds to 
pressure, and which may be thus expressed : " Stand with 
your back to the wind, and the barometer will be lower on 
your left hand than on your right." Thus expressed, how- 
ever, the law is true only for the northern hemisphere ; in the 
southern it will be reversed, the barometer being lower on 
the right hand than on the left. The same principle may 
be enunciated in another form. If you stand with the 
high barometer to your right and the low barometer to 
your left, the wind will blow on your back. The course 
of the isobars in the chart therefore indicates the direction 
of the wind, just as the distances between these lines 
indicate its strejigth?- Every Thursday the Times issues a 
weather diagram giving a graphic representation of the 
meteorology of the week, sufficiently explained, however, 
by the description which is annexed to it. 

AVhilst the Ti?fies publishes daily charts with isobaric 
curves, the other morning papers give their meteorological 
reports in different shapes. Fig. 23 is copied from the 
Daily News of to-day (March 31, 1877). It represents 

^ The arrows on Fig. 22 fly with the wind ; the asterisks indicate 
the position of meteorological stations ; and the figures give the shade 
temperature. For further information see Mr. Scott's "Weather 
Charts." 1876. 



96 



PHYSIOGRAPHY. 



[chap. 



the upper part of the scale of the barometer, ranging from 
29 inches to 30^^ inches. The height of the mercury is 
seen at a glance by the thick black lines, and we thus not 
only learn what it was in London at i a.m. on the morning 
of issue, but can compare this with the readings for the 
same hour on the three preceding days. Thus it is seen that 
on the 31st, the barometer stood at about 30*05 inches; on 
the 30th, it was 29'86; on the 29th, 29*81; and, on the 
28th, it stood at 29'58 inches. It is evident, therefore, from 




Corrected to sea^leveL, aruL reduced, to 3Z°F, 



Fig. 23. — Daily News barometer chart. 



this report that the mercury has been steadily rising. Such 
comparative readings are of great value, inasmuch as the 
character of the weather is dependent, not so much on the 
absolute height of the barometer, as on whether the mercury 
is rising or falling, and whether moving slowly or rapidly. 
It should be added that the recent barometer charts of the 
Daily Naus indicate the extreme variations of the instru- 
ment by means of dotted lines. 

The barometer chart published by the Daily Telegraph 



VI.] 



THE ATMOSPHERE. 



97 



for the same day is given in Fig. 24. This shows, by a 
graphic method, the movements of the barometer for four 
days, ending at midnight of March 30-31. The thick 
curved hne, running across the diagram, represents the 
variations of the mercurial column, and it is seen, as before, 
that the barometer has been slowly rising ; the line taking, 
in fact, a steady upward course from 29*15 to 30-06 inches. 
It should be explained that, in all these charts, the actual 
reading of the barometer has been reduced to certain 
standards, in order to secure the requisite uniformity for 




Fig. 24. — Daily Telegraph barometer chart. 

compiirison. These corrections refer to the height at 
which the instrument is placed, and to the temperature at 
which the reading is taken. It is obvious that the baro- 
i-ncter will be affected by its height above the sea-level ; for, 
as we ascend, we leave a portion of the atmosphere below 
us, and, consequently, the pressure is lessened and the 
mercury falls. Hence a barometer at the top of a house 
always reads lower than one on the basement ; the instru- 
ment is, indeed, often used for the approximate measurement 
of heights. Barometric readings from different stations 



98 



PHYSIOGRAPHY. 



[CHAP, 



cannot therefore be compared together until we know 
at what elevations the instruments are situated ; one observer 
may live on high ground and another on low, one may have 
his barometer up stairs and another down stairs. Hence il 
has come to be understood that all barometric readings 
shall be reduced to what they would be if the instrument 
were at the sea-level, which gives, of course, a fixed datum 
line. Then, again, the barometer needs correction for 
temperature. Mercury, in common with other liquids, 
expands by heat, and expands much more than the glass 
tube which holds it ; hence the barometer will rise on a 
hot day and fall on a cold day, although the atmospheric 
pressure may not have changed. It is essential therefore 
that all barometric readings should be reduced to the same 
temperature, and the standard taken for this purpose is the 
freezing-point of water, or 32° of Fahrenheit's thermometer. 
All the figures given in the newspaper charts are conse- 
quently reduced to sea-level and to 32°. 

The Standard, instead of giving diagrams or charts, 
publishes reports which, in addition to barometric readings, 
convey to the reader a good deal of useful information 
relating to the weather. This will be seen in the following 
example extracted from to-day's issue (March 31st, 1877) ; — 









Dry 

Bulb. 


Wet 
Bulb. 


During the past 24 hours. 


Date. 


Barometer p.. 
reduced to Y' % 


Max. 

Soiar 
Radia. 
in Vac. 


M^- MIn. 
14^ -r-P- 


Rain 
fall. 


Mar. 25 
„ 26 
,. 27 
„ 28 
» 29 
.. 30 


29-09 
29-79 

29'43 
29-87 
29-90 
30-13 


ESE. 

SSE 

N 

W 
NW 

NNE 


50 
47 
45 
50 
54 
51 


45 
45 
43 
47 
49 
47 


- 


55 : 44 
48 ! 38 
54 42 
57 41 
59 44 
53 48 


I-QI 
023 

o-iS 
o'47 



At Two A.M. f,h,i Barometer had risen to 30-17. 



vi.J THE ATMOSPHERE. . 99 

This gives the readings of the barometer for six consecutive 
days, taken daily at 7 p.m., and duly corrected as just 
explained. The table also gives the direction of the 
wind at the same hour each day; and indicates the humidity 
of the atmosphere, by a comparison of the wet and dry 
bulbs of Mason's hygrometer, represented in Fig. ig. It 
likewise gives the highest and lowest temperature during the 
day, and the depth of rain which had fallen in the course 
of the twenty-four hours. The column left blank, but 
headed " Maximum solar radiation in vacuo," is intended to 
receive the readings of a radiation thermometer. This 
generally consists of a delicate thermometer, having a dull, 
blackened bulb, and inclosed in a glass tube, from which 
the air has been removed. The instrument is freely exposed 
vO the heat of the sun, and its maximum reading is registered. 
The greatest amount of solar radiation which occurs during 
the day is then indicated by the excess of this temperature 
over the maximum temperature of the air in the shade. 

Although, in this chapter, the subject of atmospheric 
pressure has been dwelt upon at some length, it must not 
be supposed that we have travelled away from our specific 
purpose — the study of the basin of the Thames. It has 
been pointed out that dift'erences of atmospheric pressure 
give rise to the winds, and the character of the winds 
determines the supply of atmospheric moisture by which 
the river is fed. It is, therefore, hardly too much to say 
that, in the long run, the flow of the Thames is regulated by 
the changes in the atmosphere which are registered by the 
barometer. 

Moreover, every phenomenon of oxidation and combustion 
and the well-being, and even the very existence, of every living 
thing upon the surface of the Thames basin are absolutely 
dependent upon the composition of the air which covers it. 

u 2 



CHAPTER VII. 

THE CHEMICAL COMPOSITION OF PURE WATER. 

Had the question, " What is water ? " been asked a 
century ago, the wisest chemist of the day could have re- 
turned no answer, save that which might have been given 
thousands of years earlier. He would have replied, in 
short, that water, like air, is one of the elementary prin- 
ciples of Nature. And, yet, there had not been altogether 
wanting observations, which suggested that, after all, water 
might not be a simple substance. Thus, the sagacity of 
Sir Isaac Newton led him to infer from his optical studies, 
that water might consist ot ingredients which were unlike 
each other, and that one or more of these might be 
inflammable. Such conjectures, however, could not be veri- 
fied until considerable advance had been made in chemical 
science ; and it was reserved for the chemists of the last 
quarter of the eighteenth century, soon after they .had 
determined the composition of atmospheric air, to demon- 
strate the true chemical constitution of water. Cavendish 
and Watt in this country, and Lavoisier in France, not to 
mention other chemists, have been put forward as compe- 
titors for this honour, but the weight of evidence appears 
strongly in favour of the claims of Cavendish. With- 



CH. vii.] COMPOSITION OF PURE WATER. loi 

out entering, however, into the famous " water controversy," 
let us see what are the simplest means by which the 
composition of so common a substance may be ascer- 
tained. 

In these days of electric telegraphy every one is familiar 
with the instrument known as the galvanic or voltaic battery. 




Fig. 25. — Decomposition o£ water by electricity. 



In the year 1800, Messrs. Nicholson and Carlisle discovered 
that, when a current of electricity from a galvanic battery is 
sent through water, the liquid is at once split up into its 
constituents. Fig. 25 represents an ingenious apparatus, 
devised by Dr. Hofmann, for effecting this decomposition. 
It consis's of a U-shaped tube of glass, OH, connected 



I02 PHYSIOGRAPHY. [chap. 

with a long upright tube C, which springs from the base of 
the U. Each hmb of the U-tube has, at the top, an orifice, 
closed by a stopcock. This tube, and part of the as- 
cending tube, are filled with water, made slightly sour 
by addition of a little oil of vitriol in order to render it a 
better conductor of electricity ; but, it should be borne in 
mind, that the acid does not otherwise afifect the result of 
the experiment. In each limb of the U-tube is a piece of 
platinum, which communicates, by means of a wire, with one 
end of the galvanic battery, AB. When the battery is in 
action, a current of electricity passes in the direction indicated 
by the arrows. Starting from one end A of the battery, it 
passes through the wire to the tube O, where it enters the 
water through the platinum plate. This plate forms one of 
the electrodes ^ or entrances by which the electricity reaches 
the liquid. The current is then conducted through the 
acidulated water to the platinum electrode in the tube H, 
and thence back to the battery at B, thus completing the 
circr^.t. But, during this circuit, the current has wrought 
a curious change in the water through which it has passed. 
As soon indeed as the electric current traverses the liquid, 
streams of little bubbles rise from the two platinum plates, 
and the gases thus produced accumulate in the upper part 
of the closed tubes, whilst the displaced liquid is forced 
into the tube C, where the column consequently rises. 

These bubbles of gas result from the decomposition of 
the water. The electricity, in fact, splits the water into two 
distinct kinds of matter, both gaseous ; one gas appearing at 
tne pole where tne current enters, and the other gas where 
it leaves the water. Our apparatus enables us to collect 
each constituent separately and to examine its properties. 
On opening the stopcock at the top of the limb O, the 
* Electrode, from dSoy, hodos, a way ; otherwise called the pole. 



VII.J 



COMPOSITION OF PURE WATER. 



103 



head of water in C will force the gas out of the narrow 
orifice, and it can be examined as it escapes. On applying 
a match with its end in a state of glow, it bursts suddenly 
into flame, and burns vividly (Fig. 26), just as it did in the 
oxygen described in the last chapter ; we have in fact now 
obtained oxygen by the decomposition of water. On apply 
ing a flame to the gas which issues from the other tube, h, 
it catches fire and burns with a pale flame ; this is the gas 
which was formerly known as inflaimnable air, and is now 
called hydrogen.'' The longer the current of electricity is 




f\o. 26. — Oxygen and hydrogen from decomposition of water. 



allowed to pass, the more oxygen and hydrogen are 
generated ; and, if the current could be continued for a 
sufficient time, all the water might be thus decomposed, 
and resolved into these two gases. This experiment there- 
fore shows that pure water consists of oxygen and hydrogen. 
But it teaches more than this. You cannot fail to observe, 
from the figures, that the quantity of gas generated is not 
the same in each tube. In fact, a careful examination 
shows that twice as much hydrogen as oxygen is obtained. 

' Hydrogen, from L'Swo, hudor water ; -yivvoM, ^ennao, to produce. 



I04 PHYSIOGRAPHY. [chap. 

If a cubic inch of oxygen is generated, two cubic inches of 
hydrogen will be obtained in the same time ; and it is 
found that these proportions are exactly preserved, where- 
ever and whenever water is subjected to decomposition. 
It is seen, then, not only that water is composed of the 
two substances, oxygen and hydrogen, but that they exist 
in water in a constant proportion ; so that, when they are 
set free and assume the gaseous state, there is always one 
volume of oxygen to two volumes of hydrogen gas. 

This experiment gives a clear insight into the essential 
constitution of water. None of the changes which were 
described in preceding chapters had in any way affected 
this constitution. Water may be frozen, for example, into 
solid ice, but the ice will consist of oxygen and hydrogen 
in exactly the same proportions as in the liquid water. 
The water may be boiled and become the invisible gas, steam, 
but the steam will consist of oxygen and hydrogen in exactly 
the same proportions as in the water or in the ice. It will 
thus be understood that the physical properties of matter 
may be altered without affecting its deeper chemical con- 
stitution. The three conditions of a solid, a liquid, and 
a gas — represented respectively by ice, water, and steam 
— are physical states dependent mainly on temperature, and 
the chemical constitution of steam remains unaltered by a 
temperature far above the boiling point, while that of ice 
is not affected by any known degree of cold. It frequently 
happens, however, that the application of heat, instead 
of effecting merely physical change, produces chemical 
alteration in the substance. This was the case, it will 
be remembered, with the red oxide of mercury referred to 
in the last chapter (p. 77) ; when heated it did not melt, or 
fuse, or liquefy, but was at once spht up into its constituents 
— mercury and oxygen. And, in like manner, under certain 



vn.] COMPOSITION OF PURE WATER. 105 

conditions, water may be resolved into its elements by the 
application of heat alone, just as it may be decomposed 
by means of electricity. This interesting fact was discovered 
by Sir W. R. Grove, more than thirty years ago. He found 
that if a piece of the metal platinum, such as a small solid 
ball, be made white hot, as may be done by the intense 
heat of the oxy-hydrogen blowpipe, and then be suddenly 
plunged into water, the liquid is at once decomposed into 
its constituent gases. But although this process is of great 
theoretical interest, it is not one which in the present state 
of science can be advantageously applied to the decom- 
position of water. 

Having obtained oxygen and hydrogen by the decom- 
position of water, it may naturally be inquired whether 
these substances cannot in turn be decomposed. To this 
question it can be simply replied that the most skilful 
chemists have hitherto failed to effect such decomposition. 
They have found it impossible to obtain from oxygen 
anything but oxygen, or from hydrogen anything but 
hydrogen ; and, in the present state of our knowledge, these 
bodies are consequently regarded as elementary ox si7nple sub- 
stances. Nitrogen, which was obtained from the atmosphere 
(p. 79), is another of these elements ; and altogether chemists 
are acquainted with not fewer than sixty- four of these simple 
bodies, a large proportion of which are metals. Everything 
existing around us is consequently regarded by chemists 
either as an element or as a compowid. Oxygen, hydrogen, and 
nitrogen are elements ; carbonic acid, ammonia, and water 
are compounds. These compounds generally have proper- 
ties very different from those of their constituents ; thus, in 
none of its physical forms, does water possess the properties 
of either hydrogen or oxygen ; even as steam, it differs 
markedly from these, being neither combustible like the 



io6 PHYSIOGRAPHY. [chap. 

one, nor a supporter of combustion, like the otlier. When 
two substances are simply mixed together^ without entering 
into chemical combination, they produce a mixture, having 
properties which partake of those of its components. Thus 
if four volumes of nitrogen are mixed with one volume of 
oxygen, a mixture is obtained which resembles atmospheric 
air, and is precisely what we should expect to produce ; the 
activity of the oxygen being tempered by dilution with 
nitrogen. For this, and other reasons, chemists believe that 
atmospheric air is a ynechanical jnixture of gases ; whilst 
water is a true cJujuical compound. 

In the methods hitherto described for the decomposition 
of water, purely physical forces have been employed ; in the 
one case it was electricity, in the other case heat. But a 
similar decomposition may be effected by means of 
chemical agencies. It has just been shown that water is a 
compound of oxygen and hydrogen ; if, therefore, a body be 
presented to it which has a very strong attraction for one of 
these components, say the oxygen, it seems likely enough 
that we shall be able to draw this away, and leave the other 
constituent free. And such in truth is the case. Many of 
the metals have powerful attraction for oxygen ; and, under 
proper conditions, are capable of removing it from water, 
and thus eliminating the hydrogen. There is, for example, 
a metal well known to chemists as potassium, so called 
in consequence of its existence in common " potashes." 
Potassium so eagerly combines with oxygen that the 
moment it is exposed to the atmosphere its surface becomes 
covered with a film of oxide. Throw a small piece of 
potassium upon water, and immediately a brilliant little 
violet flame bursts forth upon the surface of the liquid, and 
darts hither and thither until all the metal is spent. By this 
means the water is broken up ; and the potassium has ciis- 



vu.] 



COMPOSITION OF PURE WATER. 



107 



placed a part of the hydrogen so energetically that sufficient 
heat has been produced to ignite the gas thus set free. 

Certain other metals closely related to potassium will also 
effect the decomposition of water, but the action is less 
energetic than with potassium. The ratidl sodii/ni, one 
of the constituents of common " soda," rends the water 
asunder, combining with its oxygen and turning out the 
hydrogen ; but the liberated gas does not catch fire spon- 
taneously, at least if the v/ater be cold. Cautiously hold a 
piece of sodium under water by means of a little wire-gauze 




Fig. 27. — jJeconiposition of water by means of sodium. 



spoon (Fig.27),and bubbles of gas will immediately arise from 
the neighbourhood of the metal ; this gas may be collected in 
a small inverted vessel filled with water, and will be found 
to burn with the characteristic flame of hydrogen. If a 
fragment of sodium be thrown on to the surface of hoi 
water,^ it is at once surrounded by flame, just as the potas 
sium was, but the flame in this case is yellow, instead of 
being violet-coloured. 

Potassium and sodium are metals but little known outside 
the chemical laboratory. The decomposition of water may, 



loS 



PHYSIOGRAPHY. 



[chap. 



however, be effected by the aid of some of the common 
metals of every-day life. Iron, for instance, answers the 
purpose well enough provided the metal is assisted by heat 
to overcome the union of the constituents of water. Fig. 
28 represents a common method of effecting the decom- 
position of water by means of iron, a is an iron tube, 
such as a gun-barrel, strongly heated in a furnace b ; water 
is boiled in the vessel c, and its vapour conducted 
through the iron tube; in traversing the heated iron tlie 




Decorapobiliun uf uatei b> means 01 ueateu 1 



steam is broken up, its oxygen combining with the iron to 
form an oxide, whilst its hydrogen is set free and may either 
be burnt directly, or collected in d. This experiment shows 
that steam, or water-gas, has the same chemical composi- 
tion as liquid water. A gaseous compound of oxygen and 
hydrogen goes in at a, and free hydrogen comes out at d; 
the oxygen having been fixed by the iron which forms an 
oxide, not indeed the same as that which exists in iron-rust, 
but an oxide identical with that which forms the natural 
loadstone, whence it is called magnetic oxide of iron. 

In these experiments the hydrogen only has been set free ; 



m.] COMPOSITION OF PURE WATER. 109 

and, in order to complete the demonstration of the chemical 
decomposition of water, it is necessary to explain how the 
oxygen may be liberated. To obtain the oxygen in a free 
form, it is clearly necessary to present to the water some 
substance which has a strong attraction for hydrogen. Such 
a substance is found in the gaseous element known to 
chemists as chlorine. This body exists largely in common 
salt, and in the well known substances " spirit of salt " and 
" chloride of lime." When set free it is an extremely 
poisonous gas, differing from all the gases to which we 
have hitherto referred, in that it possesses a very marked 
greenish-yellow colour, whence its name {xKwpog, chloros^ 
green). 

One of the most characteristic properties of chlorine is 
its powerful attraction for hydrogen. Mix the two gases 
together, and they combine with explosive violence, if 
exposed to sunshine ; and, even in diffused daylight, they 
slowly and quietly unite. This attraction for hydrogen 
stands us in good stead when we wish to get oxygen from 
water. A mixture of chlorine and steam is passed through 
a strongly-heated tube, and the chlorine eagerly seizes on 
the hydrogen to form a compound known as hydrochloric- 
acid gas, while the oxygen is set free. A similar action is 
indeed constantly being effected, in a less striking manner, 
in many of the industrial arts. Chlorine is largely used as 
a bleaching agent, but, in a dry state, it is powerless to 
bleach ; it is only when it is associated with moisture that 
it becomes active. When moist, however, it slowly 
decomposes the water, combining with its hydrogen and 
disengaging its oxygen ; and it is this oxygen, at the 
moment of its liberation, which is the really active agent in 
bleaching. 

The proof of the composition of water, derived from the 



no PHYSIOGRAPHY. [CHAf. 

action of chemical agencies, on one or other of its com- 
ponents, is now complete. On the one hand, it has been 
seen that certain metals remove the oxygen and set free 
the hydrogen ; on the other hand, chlorine removes the 
hydrogen and sets free the oxygen. Had these experi- 
ments been conducted with great care, the balance being 
used at each step, it would have been possible to determine 
the precise proportions in which oxygen and hydrogen exist 
in v^^ater. In fact, the experiment with iron furnished the 
French chemist, Lavoisier, with the means of proving the 
composition of water analytically for the first time. Suppose 
a given v/eight of water in the form of steam be passed 
through the hot-iron tube, and that the oxide of iron which 
is produced be weighed to ascertain how much oxygen 
has been obtained ; it is thus easy to tell how much oxygen 
exists in a given weight of water, and the rest will of 
course be hydrogen. In this way it has been found that 
loo parts by weight of water contain SS'Sg of oxygen and 
I I'll of hydrogen; in other words f by weight of water 
consists of oxygen, and ^ of hydrogen, so that every 9 
pounds of water contain 8 pounds of oxygen and i of 
hydrogen. This, therefore, is the composition of water 
by weight, and it agrees perfectly with what was deduced 
from our first electrical experiment with reference to the 
composition of water by vobune. It was then found, in 
dealing with bulks, that twice as much hydrogen as oxygen 
was obtained from water. Now oxygen is sixteen times 
heavier than hydrogen, taking bulk for bulk ; if therefore 
we obtained from a given quantity of water a volume of 
oxygen that weighed 16 grains, then we should find an equal 
volume of hydrogen weighed i grain ; but, as a matter of 
fact, we obtained in our experiment twice the bulk of hydro- 
gen, so that this quantity, instead of weighing i grain, must 



VII. 1 COMPOSITION OF PURE WATER. ui 

weigh 2 grains. The proportion by weight is therefore i6 
grains of oxygen to 2 of hydrogen; or 8 to i, as was 
expressed above. Chemists are thus led to the conckision 
that water consists of a combination of hydrogen and oxygen 
in the definite proportions of 2 volumes of hydrogen to 
I volume of oxygen, or of 2 parts by weight of hydrogen 
to 16 parts by weight of oxygen. 

When a com.pound is resolved into its components the 
process is called analysis^- All the processes hitherto de- 
scribed have been analytical processes, but to complete 
the discussion of the subject it is necessary to show how 
the composition of water may be demonstrated by synthesis,'' 
that is to say, by putting the constituents together and 
buildmg up the compound. The discovery of the composi- 
tion of water was indeed made originally by synthetical, 
and not by analytical, processes. 

Let pure hydrogen be perfectly dried, and then burnt : 
hold over the jet of burning gas a cold dry glass jar. Fig. 
29, and the surface becomes rapidly bedewed, the moisture 
condensing in drops which trickle down the side, and may 
be collected. These drops, are nothing but pure water, 
which has been produced by the union of the burning 
hydrogen with the oxygen of the surrounding air. Most of 
our ordinary combustibles — such as coal, wood, oil, wax, 
tallow, and gas — are rich in hydrogen, and they conse- 
quently produce water during their combustion. Hold a 
cold bright mirror near to a flame, and the moisture is 
instantly condensed upon its surface. 

Instead of the hydrogen being burnt in air, and thus 
caused to combine with atmospheric oxygen, let the hydro- 
gen be mixed with pure oxygen in the proper proportions 

^ Analysis, from ava, ana, again ; Xvctls, lusts, a separation. 
* Synthesis, from aif, sun, together ; dfo-is, thesis, putting. 



t 12 



PHYSIOGRAPHY. 



[CHAP. 



to form water. Such a mixture of gases may remain for 
any length of time, without combination being effected ; they 
will form merely an intimate mechanical mixture of the gases, 
but no water will be produced as long as there is no chemical 
union. The moment, however, that a flame is applied to 
the gases, a violent explosion occurs ; chemical combination 
is immediately brought about, the gases cease to exist as 
oxygen and hydrogen, contraction ensues, and water is 
produced. If the temperature were maintained sufficiently 
high, this water would retain the condition of steam, and it 




Fig. 29. — Formation of water by combustion of dry hydrogen. 



would then be found that every three volumes of mixed 
oxygen and hydrogen produced only two volumes of steam 
or water-gas ; in other words, contraction occurs to the ex- 
tent of one-third of the original bulk. A pint of steam consists 
therefore of a pint of hydrogen and half a pint of oxygen in 
a state of chemical combination, whereby the pint and a half 
of mixed gases becomes condensed to a pint of water-gas. 
But, at ordinary temperatures, the steam rapidly condenses 
to a liquid ; a cubic foot of steam condensing to about a 
cubic inch of water. If ox3'gcn and hydrogen be exploded 



VII.] COMPOSITION OF PURE WATER. 113 

in a dry vessel, its interior becomes bedewed with the con- 
densed moisture. 

It may seem unsatisfactory to look at a few drops of 
limpid liquid obtained by the union of oxygen and hydro- 
gen at the lecture-table, and believe that they are really 
pure water. Experiments on a large scale have, however, 
been made, and sufficient water collected to place the 
matter beyond possibility of doubt. The grandest experiment 
of this nature was made by three eminent French chemists, 
Fourcroy, Vauquelin, and Seguin. The experiment com- 
menced on the 13th of May, 1790, and was completed on 
the 22nd of the same month. During this time the ap- 
paratus was constantly watched, the experimenters sleeping 
alternately, for a few hours, on mattresses in the laboratory. 
The combustion was maintained almost uninterruptedly 
for 185 hours; 25,964 cubic inches of hydrogen and 12,571 
of oxygen were consumed, and the union of these gases 
produced 7,244 grains of hquid. With this quantity at 
their disposal, they tested the liquid in every way that 
could be suggested, and found it to be identical with dis- 
tilled water. 

There is yet another means of determining ihe chemical 
composition of water, which needs to be briefly explained, 
since it furnishes the most accurate of all methods for 
determining this important question. Transmit a stream of 
pure dry hydrogen gas over a weighed quantity of pure oxide 
of copper (a compound of oxygen and the metal copper) 
heated to dull redness. Under these circumstances the 
hydrogen takes oxygen from the oxide, and forms water, 
which can be collected and weighed, whilst the loss of 
weight which the oxide suffers must represent the quantity 
of oxygen which this amount of water contains. It is 
needless to enter into the details required to secure accuracy 



114 PHYSIOGRAPHY. [CH. vii 

in so delicate an experiment ; but, in the hands of modem 
chemists, it has yielded the most trustworthy results which 
we possess on this subject. These results agree substan- 
tially with those which have been stated before. Indeed 
there is no fact in the whole range of chemical science 
better established than this, — that water is a definite 
chemical compound of oxygen and hydrogen, in the pro- 
portions by weight and measure previously given in this 
Chapter. 

And thus we arrive at the last word of science, in its 
present state, respecting the origin of the river Thames. So 
far as its flood is pure water, we can trace that water to the 
ocean. And the pure water which forms the chief com- 
ponent of the sea has certainly been formed, some time or 
other, by the union of two bodies, oxygen and hydrogen, 
which, in their free state, are known only in the physical 
condition of gases. 



CHAPTER VIII. 

THE CHEMICAL COMPOSITION OF NATURAL WATERS. 

Our study of the chemical constitution of water, in the 
last Chapter, led to the conclusion that this liquid consists 
of two gases, oxygen and hydrogen, united in definite 
proportions. Such indeed is the composition of absolutely 
pure water, but such is not the composition of any water 
known upon the surface of the earth. As a matter of fact, 
absolutely pure water is never found in the economy of 
nature. The great stream of water called the Thames is 
far from being absolutely pure in any part of its course. 
In the neighbourhood of the metropolis, it is, as every one 
knows, contaminated with impurities to such an extent as to 
acquire its proverbial turbidity. The muddiness, however, 
is due to the presence of solid particles which are mechani- 
cally suspended in the water — particles which would in 
great measure subside if the water were left free from dis- 
turbing causes, and which could be more or less completely 
removed by the simple process of filtration. Apart, however, 
from these mechanically suspended impurities, the Thames 
water, like the water of every other river, contains certain 
chemical compounds in a state of solution. Such impurities. 

1 2 



Ii6 PHYSIOGRAPHY. [chap. 

though present in very large proportions, may entirely 
elude observation by the eye, the water remaining clear and 
colourless. These soluble constituents, unlike the suspended 
impurities, will not be deposited when the solution is allowed 
to stand, nor will they be removed by the mere act of 
filtration. All natural water — whether brook or river, lake 
or sea — contains such dissolved matter, chiefly in the form 
of the various compounds called salts ; but it varies con- 
siderably, in character and in quality, in different varieties 
of natural waters. 

It is not necessary to go far to seek the source of these 
dissolved impurities. All the rocks of the earth, over 
which the waters flow, or through which they drain, contain 
mineral constituents more or less soluble in water. Water 
is, in fact, an almost universal solvent, whether of solids, 
liquids, or gases. River- water must therefore be regarded, 
not as absolutely pure water, but rather as an extremely 
weak solution of certain chemical compounds. What these 
compounds are will now be explained. 

When natural water is evaporated, all its impurities, 
except those which are volatile, are left behind, and the 
vapour which rises is very nearly pure water. When the 
vapour of water is condensed it reproduces pure water. But 
such water rapidly absorbs both oxygen, nitrogen, carbonic 
acid, and ammonia ; and, hence, the rain when it reaches 
the earth is no longer pure water ; it has absorbed some of 
the atmospheric gases. Rain-water, therefore, although the 
purest of all forms of natural water, contains certain im- 
purities which it has washed out of the atmosphere. The 
oxygen of the air is more soluble than the nitrogen ; the 
carbonic-acid gas is much more soluble than either oxygen 
or nitrogen ; and the ammonia is far more soluble than 
any of the other gases. Thus, under normal conditions of 



viil.J COMPOSITION OF NATURAL WATERS. 117 

temperature and pressure, 100 volumes of water dissolve 
I '48 volumes of nitrogen, 2'99 volumes of oxygen, ioo"2 of 
carbonic acid, and 78270 of ammonia. All the constituents 
of the atmosphere will therefore be found in a state ol 
solution in rain-water ; while oth§r bodies, such as nitric 
acid, also derived from the atmosphere, are not unfrequently 
present. In fact, whatever soluble constituents exist in the 
air will be absorbed by the rain. Hence, in the neighbour- 
hood of towns, where the atmosphere is impure, the rain- 
water will wash out more or less of its impurities, and, 
consequently, the rain collected in populous districts is less 
pure than that collected in an open country. Moreover, the 
rain which falls at the beginning of a shower is more 
contaminated than the later rain ; and rain, which falls 
after a long drought, is more impure than that which falls 
towards the close of a rainy season. But, even after a long 
continuance of wet weather, the rain will still contain 
atmospheric gases to the extent of about 2^ cubic inches 
to 100 cubic inches of water. 

When the rain reaches the surface of tne earth, it im- 
mediately commences to attack the rocks on which it happens 
to fall. Whether it dissolves much or little will depend 
upon whether the earth contains more or less soluble 
matter. But, whatever the character of the ground, some- 
thing will certainly be always dissolved. Every runnel, 
every brook, every rivulet, thus washes out some of the 
soluble constituents of the rock over which it flows, and 
carries them onwards to the river. The river consequently 
becomes the common receptacle for all the soluble matter 
delivered by its tributary streams. As it flows along, it 
grows richer in these soluble constituents, deriving them 
partly from the wear of its own bed and partly from that of 
its banks. It is not, however, by merely running over the 



ii8 PHYSIOGRAPHY. [chap 

surface of the ground that the river and its affluents de- 
rive their supply of soUible impurities, much more being 
probably due to the springs by which the streams are 
chiefly fed. Spring-water is, in fact, much richer than river- 
water in these soluble constituents. Nor is it difficult to 
see why. 

To form a spring, the rain-water must sink to a greater or 
less depth in the ground. During its underground passage 
it exerts its solvent action upon the surrounding rocks. In 
some cases, the water sinks to great depths, traversing long 
and tortuous passages ; and, in such cases, it is only to be 
expected that, when it reappears at the surface, it will be 
highly charged with soluble constituents. Under pressure, 
at great depths, it may absorb large volumes of such gases 
as carbonic acid and sulphuretted hydrogen ; or it may dis- 
solve saline matters of various kinds, and thus acquire 
peculiar properties which confer upon it medicinal value. 

Analysis of the water of the Thames Head Well, near 
Cirencester, shows that it contains 2 7 '44 parts of solid im- 
purity dissolved in 100,000 parts of the water ; in other 
words, 0*02744 per cent. The most notable of the mineral 
constituents which affect the quality of the springs in the 
Thames basin is carbonate of lime. 

A large part of the course of the river lies, indeed, 
through limestone rocks. In the upper part of the basin, 
it is the limestones of the Oolitic formations that furnish 
most of the springs ; whilst, in the lower part, it is chiefly 
the chalk. All limestones, from the softest chalk to the 
hardest marble, consist essentially of carbonate of lime ; 
and as this compound is slightly soluble in water, the springs 
and streams of limestone districts always hold it in solution. 
It is true that the proportion of carbonate of lime dissolved 
by pure water is extremely small; not more, it is said, than 



VIII.] COMPOSITION OF NATURAL WATERS. 119 

two grains in a gallon of water. But, when water is charged 
with carbonic acid gas, the carbonate of lime is dissolved 
with facility; and, since most spring-water contains this gas, 
it is easy to understand how it can act with great effect upon 
limestone rocks. It has been seen that carbonic acid gas 
is dissolved out of the atmosphere by rain-water ; and, in 
like manner, every piece of water exposed to the air must 
absorb it. Hence, all natural waters can dissolve carbonate 
of lime, with more or less ease, and thus erode the lime- 
stone rocks through which they drain. 

When such calcareous waters are used for domestic pur- 
poses, they are found to curdle soap, and are consequently 
termed //ar^ waters. A portion of the soap is wasted, inas- 
much as its fatty acids form insoluble salts with the lime. 
As long therefore as this curdling continues, the soap is being 
wasted, and a lather cannot be produced. It is clearly con- 
venient to have some means of comparing the relative hard- 
ness of different waters. To this end, Dr. T. Clarke, many 
years ago, proposed a scale in which each degree corresponds 
to one grain of carbonate of lime in a gallon of water. 
According to this scale, the water of the Thames Head 
Well has a h-ardness of 23 degrees ; that is to say, it 
contains salts of lime equivalent to 23 grains of carbonate 
to the gallon,^ But it is possible to improve the condition 
of such hard water by an easy process of softening, intro- 
duced likewise by Dr. Clarke. This consists in simply 
adding lime-water to the water the hardness of which is to 
be corrected ; the lime combines with the excess of carbonic 

^ The imperial gallon contains 70,000 grains. In many official reports 
the proportion of carbonate of lime is given, not as grains per gallon, 
but as so many grains in 100,000 grains of water. The conversion 
of one form of result into the other is of course a mere question of 
proportion. 



(20 PHYSIOGRAPHY. [chap. 

acid to form the almost insoluble carbonate of lime ; which 
is then precipitated, in company with the original carbonate 
of lime, thus rendered insoluble by removal of the carbonic 
acid that held it in solution. In this way, the hardness of 
the water of Thames Head Well may be reduced from 23° 
to 5°. Such a process of softening is carried on upon a 
large scale at several water-works, as at Caterham and at 
Canterbury. 

The hardness which is thus capable of correction is 
termed temporary hardness, to distinguish it from that which 
cannot be removed by treatment with lime, and which is 
consequently termed permanent hardness. Such permanent 
hardness is due to the presence of sulphate of lime. It may 
therefore be said that the water of Thames Head has a hard- 
ness of 23°, of which 18° represent temporary hardness and 
5° permanent hardness. The upland waters of the West of 
England usually contain much more sulphate than carbonate 
of lime. Sulphate of lime occurs crystallized in nature, 
and is known to the mineralogist under the rather fanciful 
name of Selenite^ whence waters containing much sulphate 
of lime are termed selenitic waters. If a water be described 
simply as calcareous^ it is generally assumed that the parti- 
cular salt of lime which it holds in solution is the carbonate. 

Waters flowing through limestone districts are generally 
charged with this salt \ in many cases to so great an extent 
that, if the water be exposed to the air, the carbonate of 
hme is spontaneously thrown down in a solid form. Such 
springs are vulgarly ca\\Q(\ petrifying springs. To "petrify," 
however, means literally to turn into stone ; it should there- 
fore be distinctly understood that all such springs are able 
to do is to simply cover the objects which receive the water 
with a crust of carbonate of lime, and not actually to con- 

' Heloiite, from ciK-nvn, selene, tlie moon. 



VIII. j COMPOSITION OF NATURAL WATERS. izi 

vert them into mineral matter. Thus, at Matlock, in Derby- 
shire, the water flowing through the Carboniferous Limestone 
is caused to deposit its carbonate of lime upon various 
objects exposed to its action, and in this way the so-called 
petrified bird's nests and other curiosities are produced. 
Thick deposits of carbonate of lime are frequently formed 




Fig. 30. — Bridge of travertine at Clermont, Auvergne (Scrope). 

by calcareous springs where tliey issue into the air. Fig. 30 

represents a natural bridge of carbonate of lime formed 

by calcareous water at Clermont in the Auvergne, described 

many years ago by the late Mr. Poulett Scrope.'' Here, the 

^ TAe Geology and Extinct Vclcauoes of Central FranCt'. By G. 
Poulett Scrope, M.P., F.R.S. Second Edition, p. 22. 1858. (By 
permission of Mr. Murray.) 



fi2 PHYSIOGRAPHY. [CHAr. 

water has formed for itself an aqueduct, 240 feet in length, 
which terminates in a large arch spanning the stream into 
which the water at one time flowed. All this solid mass 
must have existed originally in an invisible state of solution 
in the water of the spring. Such deposits of carbonate 
of lime are commonly termed travertifie, a word supposed 
to have been derived from the old name. Lapis Tibiirtimis, 
which was formerly applied to the stone, in consequence of 
its deposition on a large scale from the calcareous waters of 
the River Anio at Tivoli, the ancient Tibur, near Rome. 
At the Falls of the Anio, the travertine has formed bed after 
bed, to the thickness of four or five hundred feet. 

In consequence of the comparative ease with which 
limestone yields to the solvent action of water holding 
carbonic acid gas in solution, this rock is frequently worn 
by water into holes and caverns. When calcareous water 
finds its way into the roof of a cavern it slowly deposits its 
burden of carbonate of lime, or at least a portion of it, in 
a solid form ; and, by long continuance of this action, ulti- 
mately produces a conical or cylindrical body hanging like 
an icicle from the rocky roof. Pendent rods of this kind 
are termed stalactites} From the point of the stalactite, 
water slowly drops down upon the floor, and, as this water 
likewise contains carbonate of lime, another calcareous 
deposit is formed as a little conical mass seated on the 
floor ; this mass is termed for distinction sake, a stalagmite.'^ 
As the stalagmite grows in height, it approaches the stalactite 
above, which continues to grow downwards ; and, ultimately, 
the two may meet and thus form a solid pillar stretching 
from floor to roof. Fig. 31 will give some idea of the com- 
mon shapes assumed by stalactites and stalagmites. It 

^ Stalactite, from (rra\d(raoo, stalasso, to drop. 
* Stalagmite, from uTaXayi-ia, stalagma, a drop. 



VIII.] COMPOSITION OF NATURAL WATERS. 123 

represents a cavity, described by Professor Boyd Dawkins 
as the Fairy Chamber, in a Hmestone cavern on the isle of 
Caldy opposite to Tenby in Pembrokeshire.' In the forma- 
tion and decoration of such caves, water is the main agent 
from beginning to end. Finding its way through the cracks 






Fig. 31.— Stalactites and stalagmites, Isle of Caldy. 

and crannies of the sohd rock, it first eats away the hme- 
stone so as to form the cavity, and then bedecks the roof, 
the floor, and the walls, with calcareous deposits of most 
fantastic shapes. Even without going into a limestone 

^ Cave Hunting. By W. Boyd Dawkins, M.A., F.R.S. P. 64. 
«S74. 



124 PHYSIOGRAPHY. [chap. 

caverns, examples of these stalactites may readily be seen. 
In fact, it is by no means uncommon to see small stalactites 
lianging down, like icicles, from the roof of the arches of 
a railway bridge, where they are produced by the rain-water 
dissolving the calcareous matter contained in the roadway, 
or in the materials of which the arch is composed. 

Calcareous salts, although the most common, are by no 
means the only mineral compounds which occur in natural 
waters. Some springs, such as those at Epsom, are rich in 
sulphate of magnesia ; whence this salt is popularly called 
Epsom salts, while the springs themselves are said to be 
saline. Others may contain salts of iron, and form chalybeate 
springs, as mentioned at p. 26. It is notable that many 
mineral springs have a temperature higher than that of the 
locality in which they issue : thus the warm springs of Bath 
have a temperature of nearly 120° F. In volcanic districts, 
such thermal sources are extremely common, and as water 
when hot dissolves most substances more freely than when 
cold, these springs are often rich in mineral matter. The 
famous geysers of Iceland and of Colorado are intermittent 
boiling springs, containing in solution a good deal of silicia, 
or the matter of which flint and rock crystal are composed. 
(See Chapter XIII.) 

Springs, such as have been referred to above, are of course 
exceptional ; but, it should be remembered, that all spring 
water contains more or less mineral water in solution. 
On comparing the composition of river water with that of 
spring water, it will generally be found that the river con- 
tains less saline matter. In fact, the water discharged into 
the river by springs becomes diluted by direct influx of 
rain, and this dilution more than compensates for loss by 
evaporation ! so that, on the whole, the proportion of salts 
diminishes. Moreover, the organisms inhabiting the river 



VIII.] COMPOSITION OF NATURAL WATERS. 125 

derive their needful supply of mineral matter, directly or 
indirectly, from the surrounding medium, and thus the 
fresh-water shell-fish and crustaceans appropriate a large 
quantity of carbonate of lime, to form their shells, from 
the river in which they live. Much of this, however, must 
be returned to the river by the decay of the shells after the 
death of the animals. In these and other ways, it is easy 
to account for the proportion of saHne constituents being 
less in river than in spring-water. If the river drain a 
country composed of hard and almost insoluble rocks, the 
water will contain but little mineral impurity. Thus the 
water of the Dee, of Aberbeen, which draws its supply from 
a granite district, contains only about three grains of saline 
matter in the gallon. It is a very different case, however, 
with a river like the Thames, which collects its water from 
the drainage of comparatively soft and soluble rocks. The 
composition of the Thames water maybe seen by the follow- 
ing analysis : — ^ 



COMPOSITION OF THAMES WATER AT LONDON BRIDGE IN GRAINS 
PER GALLON OF 70,O0O GRAINS. 



Carbonate of Lime 
Chloride of Calcium ^ 
Chloride of Magnesium . 
Chloride of Sodium 
Sulphate of Soda . 
Sulphate of Potash 
Silica .... 
Insoluble Organic Matter 
Soluble Organic Matter . 



S-II65 

6-9741 

■079S 

2-3723 

3-1052 

•2695 

•1239 

4*6592 

2 "3380 

28-0385 



* "Analysis of Thames Water." By John Ashley. Quarterly 

Journal of the Chemical Society, vol. ii. p. 74. 

° Probably the calcium exists rather as sulphate of lime, and the 
chlorine as chloride of sodium. 



126 PHYSIOGRAPHY. [chap. 

Although the proportion of mineral matter held in 
solution in Thames water appears, from such an analysis 
as that just cited, to be extremely small, it must yet be 
remembered that, taking into consideration the great 
volume of the Thames, the total quantity of matter 
removed in this way from the land and carried seaward 
is something enormous. Professor Prestwich, taking the 
•daily discharge of the Thames at Kingston at 1,250 
million gallons, and the salts in solution at 19 grains per 
gallon, calculates that the quantity of mineral matter carried 
down in solution, at that locality, amounts to 3,364,286 
pounds, or 1,502 tons, every twenty-four hours; or say, 
roughly, a ton a minute. Of this amount about 1,000 tons 
will consist of carbonate of lime, and 238 tons of sulphate 
of lime. The total quantity of saline matter carried in- 
visibly away by the Thames from its basin above Kingston 
will thus reach, in the course of a year, to the enormous 
amount of 548,230 tons. 

Although it has been shown that river-water contains 
a smaller proportion of saline matter than is present in 
spring-water, it would yet be a great mistake to assume 
on this ground that the river-water is more pure and whole- 
some. On the contrary, the river-water, though poor in 
mineral matter, is usually rich in organic impurities, and 
much less fitted for drinking purposes. Most of the 
water from deep wells and springs in the Thames basin 
contains the merest trace of organic matter; but the 
river itself derives a large proportion of organic impurity 
from the decomposing vegetable matter, spread over the 
large surface of country which it drains. A more serious 
source of contamination, however, is to be found in 
the sewage which is allowed to run into it from the 
centres of population seated on or near its banks, and 



VIII.] COMPOSITION OF NATURAL WATERS. .127 

which may be charged with matters capable of propagating 
disease. In a shallow stream, which flows rapidly over an 
irregular bed, and is thus thoroughly aerated, the organic 
impurities derived from decaying animal and vegetable 
substances may be gradually destroyed by oxidation. The 
mineral substances in solution, on the other hand, remain 
unaffected, save in so far as they may be consumed in 
supplying such matters to the organisms which inhabit 
the river. The mineral salts are therefore, for the most 
part, borne onwards by the river and finally discharged 
into the sea. The sea consequently becomes the ulti- 
mate receptacle for all the saline matter washed out of 
the land and brought down by rivers. And yet the 
water of the sea differs considerably in chemical com- 
position from that of rivers or of springs. Whilst a 
gallon of Thames water contains in solution about 21 
grains of saline matter, a gallon of sea-water will contain 
something like 2,400 grains. In fact, the proportion 
of solid matter in sea-water reaches as high as 3^ to 4 
per cent. It is well known that most of this saline matter 
consists of common salt, such as we use at table, — 
a salt known to chemists as chloride of sodium, since it 
consists of two elements, namely, the gas chlori7ie and the 
metal sodium. Out of the 2,400 grains of mineral matter 
in a gallon of sea-water, nearly 2,000 grains will consist ot 
this common salt. 

As an example of the composition of sea-water, the 
following analysis^ of water from the British Channel may 
be quoted. The density of this water was found to be 
1027. 

^ An analysis, by Schweitzer, in the Philosophical Magazine, vol. xv. 
p. 58. Recalculated to bring it into comparison with the analysis of 
Thames water on p. 125. 



,28 PHYSIOGRAPHY. 'CHAi 



COMVOSITION OF WATER OF THE KRITISH CHANNEL IN GRAINS 
PER GALLON OF 70,000 GRAINS. 



Chloride of Sodium . 


1964-165 


Chloride of Potassium 


53-585 


Chloride of Magnesium 


256-655 


Bromide of Magnesium 


2-044 


Sulphate of Magnesia 


16-069 


Sulphate of Lime 


98*462 


Carbonate of Lime 


2-310 


Iodine and Ammonia . 


traces 




2393-290 



Every tide brings this sea water into contact with the 
fresh water of a tidal river, Hke the Thames, and the two 
kinds thus become mixed. On going down the Thames 
from London Bridge, it is found that the water gradually 
loses its freshness. A little below Gravesend, it begins to 
acquire a saltish flavour, and this saltness increases until the 
water becomes decidedly brackish and undrinkable. Going 
still farther out into the estuar)"-, the saltness becomes more 
pronounced ; and, by the time Whitstable is reached, the 
water- is hardly to be distinguished from that of the sea 
itself. 

I The fresh water brought down by a river does not how- 
ever immediately mix with the salt water, but rather tends 
to float upon its surface. For, since the sea water is rich in 
solid matter, its density is proportionally high ; that is to say, 
sea-water must weigh considerably more than fresh water 
when equal bulks are compared. A gallon of water from 
the Nore weighs rather more than a gallon of water from 
the Thames at Teddington, If a given measure of pure 



Vlll.j COMPOSITION OF NATURAL WATERS. 129 

water weighs 1,000 lbs. the same measure of water from 
Margate will weigh 1,027 lbs. As a consequence of this 
high density, it is easier to swim on salt than on fresh 
water. Hence, too, the fresh water carried down by a river 
tends to float for a time upon the surface of the Jense sea- 
water ; and, off the mouths of some great rivers, the water 
is found to be nearly fresh for some distance out to sea. 

From the vast surface exposed by the sea, water is con- 
tinually being evaporated by the aid of solar heat. But it is 
practically pure water which is thus drawn up into the 
atmosphere, the saline constituents of the sea water being 
left behind. Pure water condenses from this vapour, and 
falling upon the land as rain, charged to a certain extent with 
the constituents of the atmosphere, it washes out more or less 
of the soluble constituents of the rocks, which are ultimately 
carried down to the sea, where they accumulate. There is 
consequently, a never-ceasing transference of solid matter 
from the land to the ocean — a transference, however, which 
entirely escapes cognizance by the sight, since the matter 
is carried down in a state of invisible solution. But, as 
was remarked at the commencement of tliis chapter, in 
addition to the dissolved mineral matter which thus eludes 
observation, the Thames, like other rivers, bears a vast 
quantity of other solid matter in a state of mechanical 
suspension, and therefore readily recognized by the eye. 
This mechanical transport of solid matter from earth to sea 
will forna the subject of the next Chapter. 



CHAPTER IX. 

THE WORK OF RAIN AND RIVERS. 

Take a gallon or two of water out of the Thames at 
London Bridge, and allow it to stand quietly in a clean 
vessel. If you look at it, after it has stood for several 
hours, you will find that the water is much clearer, and 
that a quantity of muddy matter is spread over the 
bottom of the vessel, the quantity being greater or less 
according to the condition of the river at the time you 
happen to examine it. This mud was previously held in 
suspension by the water, and was the main cause of its 
turbidity, so that, as soon as the muddy particles settled, 
the water became clearer. While the water was in the 
river, the fine solid particles were kept in constant agita- 
tion by the current of the stream, and were thus prevented 
from settling dov/n. The more rapid the stream, the greater 
is its power of carrying this suspended matter ; but, as the 
river approaches its mouth, the flow becomes slackened and 
the sediment subsides. Hence, in the lower part of the 
course of the Thames, especially in the " reaches," or bends 
in the river, near Woolwich, there are large mud banks ; 
and this mud is systematically dredged up and removed, in 
order to prevent obstruction. Those particles of mud which 



CH. IX.] THE WORK OF RAIN AND RIVERS. 131 

are very light may be kept suspended in the water until they 
are carried by the river right out to sea ; but a time at length 
comes, when even these will quietly settle down upon the 
sea-bottom. If a little of the muddy sediment, deposited by 
the water, be dried by exposure to the air, it will be found 
to harden into a substance not unlike clay. Clay is, in 
fact, nothing but such mud, hardened and perhaps otherwise 
altered. 

Very iittle thought is necessary to convince any one that 
the fine particles of solid matter, which form mud, are pro- 
duced by the mechanical waste of the land. After a heavy 
shower of rain has fallen in the street, you observe dirty 
streams coursing along the gutters, and every one knows that 
the muddy matter in these streams is merely the dirt washed 
from the roofs of the houses and the stones of the street. 
In like manner, every shower of rain that falls in the open 
country washes something off the surface of the land. This 
removal of matter is termed deinidation, since the rocks are 
laid bare by having their superficial covering thus peeled off. 
The particular kind of denudation effected by means of rain 
is called ////w'cz/^ denudation. A heavy shower, falling upon 
a field, washes away some of the soil, and carries it off in 
muddy runnels to the nearest stream, whence it passes to 
the river. Where the rain comes down in a deluge, as often 
happens in the tropics, its power as a denuding agent is 
almost incredible ; and even in this country, especially 
among the hills of Wales and Cumberland, we occasionally 
hear of torrents of rain tearing up rocks and sweeping 
everything before them. 

The detrital matter which is worn away from the land, 
and carried along by rivers, contains materials of every 
degree of coarseness. It often happens that fragments of 

' Pluvial, from the T,at. phivia, rnin. 



132 PHYSIOGRAPHY. [chap. 

rock, perhaps of considerable size, are loosened from cliffs 
near a river by the action of rain and frost, and tumble 
down into the stream. There they get slowly worn down, 
by constantly knocking against each other, and may ulti- 
mately be rubbed into the form of smooth round pebbles. 
In the basin of the Thames, it is common for the hard 
flints from the chalk to get broken and rolled about 
in the water, and it is in this way that gravel is formed. 
The substance which is strewn over our roads and 
garden-walks consists, chiefly, of little bits of flint, which 
have been so rolled about in water that the sharp points 
of the broken stones are rounded off. All gravel has 
not however been subject to the same amount of rough 
usage, so that whilst the pebbles are in some cases 
well rounded, in other cases they retain more or less 
of their angularity, though the corners are never quite 
sharp. The small pieces worn oft' the fragments of rock, 
as they rattle together on the bed of the stream, get 
rolled about until they form small rounded grains known 
as satid. As a rule, both the gravel and the sand 
consist, chiefly, of the substance called si/ica, or the 
material of which flint is formed, and which is chemically 
the same as the matter of pure rock-crystal (p. 58). 
The gravel and the coarser sediment are pushed along 
the bottom of the river by the motion of the stream, 
whilst the finer sand may be carried in suspension, 
though it will not travel so far as the lighter particles 
of mud. The heavier pieces naturally fall to the bottom 
first, so that if a quantity of mixed gravel, sand and 
mud be shaken up in water, it will be found that the 
gravel is the first to fall ; then the sand subsides, and 
finally the mud settles down. 

If a river has a steep bed it generally possesses great 



IX.] THE WORK OF RAIN AND RIVERS. 133 

carrying power. Mountain-torrents, for example, rush 
down steep slopes and not only transport vast quantities 
of gravel, sand and mud, but often move stones of 
considerable weight. During floods, too, ordinary rivers 
acquire great mechanical power. Thus we read of floods 
sweeping away bridges, tearing up rocks , from the banks 
of the river, and carrying along stones weighing several 
tons. Sir T. D. Lauder, in describing the great floods 
which occurred in Morayshire in August 1829, records 
the destruction of many farms and hamlets ; while no 
fewer than 38 bridges were swept away by the flooded 
rivers. A huge mass of sandstone, measuring 14 feet 
in length, 3 feet in width, and i foot in thickness, was 
carried for a distance of 200 yards by the swollen stream 
of the river Nairn. 

In estimating the carrying-power of running water, it must 
be borne in mind that the weight of a stone is considerably 
less in water than in the atmosphere. When a body is im- 
mersed in water, it appears to lose a certain proportion of 
Its weight, the proportion depending upon its specific 
gravity. If a stone be twice as heavy as an equal bulk 
of water, it will lose one-half its weight ; if three times as 
heavy, it is lightened by one-third ; and so on. It is 
generally said that if a stream flow at the rate of six inches 
per second it has power enough to carry off fine sand ; if 
at 12 inches per second it can sweep away fine gravel; and 
if at 36 inches per second it can transport pebbles as large 
as hen's eggs. It should not be forgotten, however, that the 
shape of the fragments greatly affects the ease with which 
they may be moved in water. 

Hitherto, the work of the river has been regarded as 
chiefly that of transporting solid matter which has been 
carried into it by rain and other denuding agents. But the 



(34 PHYSIOGRAPHY. [chap 

river is, itself, a powerful agent of direct denudation — 
fluviatile denudation as it is sometimes termed. It is true 
that running water, alone, can do but little towards abrading 
a hard rock : but the pebbles, sand and other detrital matter 
carried along by the stream, rub against every hard point 
with which they come in contact, and thus enable the river 
to wear away the hardest rocks in its course, as surely as 
though they were being ground and scoured with sand- 
paper. The grinding action of pebbles, when set in motion 
by water, is strikingly shown in the formation of potholes. 
These are roundish cavities, perhaps several feet in depth, 
not uncommon in the hard bed of a mountain-stream. A 
few pebbles, lodging in a small cavity, get whirled round and 
round by the eddies of the stream, until, at length, they 
excavate deep holes of considerable size. In such cases, the 
grinding effect of the pebbles is generally assisted by the 
sand and finer particles in the water, which scour the walls 
of the hole as effectually as though they were well rubbed 
with fine sand-paper. 

Aided by its burden of detrital matter, the river frets 
away the rocks along its banks and thus tends to widen its 
channel; while, at the same time, the coarse sediment scratch- 
ing along the bottom ^ helps to tear it up and thus deepen 
the bed of the river. Every stream, with sufficient fall, is 
in this manner continually at work, gnawing away the rocks 
through which it flows, so that a channel which is, to begin 
with, narrow and shallow may gradually become widened and 
deepened. The amount of excavation which can be wrought 
in a given time, by means of running water, is well seen in 
volcanic regions, where rivers have cut through sheets of 
lava which have been poured forth at known dates. 

But perhaps the grandest results of river-denudation are 
to be witnessed in the vast chasms through which some of 



tX.] THE WORK OF RAIN AND RIVERS. 135 

the rivers in Colorado flow. These narrow gorges, bounded 
by steep wall-like cliffs, are known under their Spanish 
name of cafions (Fig. 32). ^ The Colorado River of the 
West, which runs from the Rocky Mountains to the Gulf 
of California, flows, during part of its course, at the bottom 
of a profound chasm ; being hemmed in by vertical walls 
which, in some places, are more than a mile in depth. 
There is no reason to doubt that this gigantic furrow has 
been cut down by the river which runs through it. The 
tributary streams flowing into the river run, in like 
manner, through smaller ravines, known as side-canons ; 
and, in fact, the general arrangement of the canons at 
once suggests that of the drainage system of a country. 
Nothing can show the amount of vertical erosion, effected 
by running water, better than these gorges. Probably 
they owe the preservation of their peculiar form to the 
fact that the country in which they occur is comparatively 
rainless ; for, if there were much rain, the sides could 
not retain their position as perpendicular walls, and 
denudation would gradually convert the chasm into an 
ordinary river valley. 

To understand how running water usually effects denuda- 
tion, it is instructive to watch, at the sea-shore, the 
behaviour of the water which drains off a flat coast of 
mud, or fine sand, as the tide retreats. Flat and smooth 
as the beach may seem to the eye, the water soon finds 
out some slight inequalities of surface, and runs down 
even the gentlest declivity. Particles of sand carried 
down by the water begin to scour out little grooves 
and then to enlarge them into wider furrows. Several 
streams may be seen uniting into one larger stream, and, 

1 From Powell's Exploration of the Colorado River of the JFest. 
Washington, 1875. 



136 PHYSIOGRAPHY. [CH ix. 

at length, a complex system of branches is established, 
all tending to a common channel v/hich runs down 
towards low water. Even without going to the sea-side, 
one may often see similar effects near a way-side puddle 
which receives the muddy drainage of the road. No 
imagination is needed to compare the miniature system 
of branching streams, produced in either of these cases 
under one's eyes, with the drainage system of a river basin. 
The model is in fact complete in almost every point. 
There is the main stream, with its side feeders, running 
down to the sea; and, it may often be seen, that one 
little system of streams is separated from another by an 
intervening space which represents a water-parting. 

Suppose now that a portion of the sea-bottom were to be 
upheaved, and appear above the surface of the water as a 
great mud-flat. From what has just been said, it is easy to 
judge at once how it would be drained. When rain fell 
upon this new-born land, it would be sure to find some 
slight rise and fall of the surface, and the gentlest fall is 
sufficient to determine that the rain shall run in this direc- 
tion rather than in that. The very fall of the pattering 
rain-drops would produce little dimples on moist ground, 
and thus give rise to superficial irregularities. As the water 
flowed off in runnels, it would wash away fine particles of 
the mud ; and thus every shower would find better channels 
scooped out to receive the drainage. The streams would 
certainly not run down to the sea in parallel straight lines, but 
a number of neighbouring streams, all tending to the lowest 
level, would soon be gathered together in a common channel 
something like that shown in Fig, ;^^. If the action went 
on for a long time, the water-channels would get worn wider 
and deeper, while the sides of the streams would be washed 
by the rain into sloping banks. So close indeed is the 




Fig. 32. — The Grand Canon, Colorado. 



>3S 



PHYSIOGRAPHY. 



[CHAf>. 



similarity of a system of drainage established in this way 
to what is found in a large river basin, that those who have 
thought most upon the subject believe that one may be 
taken to explain the other; that, in point of fact, the pre- 
sent rivers have gradually scooped out their own channels, 
and that our river-valleys are, mainly, the result of work per- 
formed by rain, rivers, and similar agents of denudation. 




Fig. 33. — Self-established dniinage system. 



At first sight, it may seem incredible that a great river- 
system, like that of the Thames, should have been shaped 
by the action of instruments which seem so insignificant. 
Yet the more one thinks upon it the less are the difficulties 
that beset such an explanation. No one can deny that 
little water-courses may be eaten out of solid rock by a 
running stream, for the very origin of such guUeys may 



IX.J THE WORK OF RAIN AND RIVERS. 139 

often be witnessed. And, from these, it is possible to 
pass, by insensible steps, to brooks and streams of larger 
size, until, at length, you come to a true river. If it be 
admitted that the little stream has worn out the gutter 
in which it runs, it is hard to deny that the larger stream 
has not done similar work on a larger scale. The whole 
affair is indeed a mere question of time. The smallest 
cause can produce a vast effect if it is only allowed to 
work long enough. 

It needs but little boldness to apply such reasoning 
to the valley of the Thames. On looking at the two 
opposite sides of the valley it may often be seen that 
the rocks exactly correspond ; a bed of gravel on one 

Wiinbledou ' Valley of tlie IVcindsworth Clapham 

Coinmcn. IVandle. CotinnoH. Common. 



Fig. 34. — River-valley worn through gravel and London clay. 

bank, perhaps, has its counterpart on the other. Fig. 
34 is a section from Wimbledon Common to Clapham 
Common.* Here it is seen that the surface of Wim- 
bledon Common is covered by a bed of gravel spread 
over the London clay. On descending from the summit of 
the common to the valley of the river Wandle, the gravel 
is seen to be abruptly cut off and succeeded by the 
underlying clay ; but, on going up the opposite slope of the 
valley, the gravel again appears, at about the same level, and 
covers the surface of Wandsworth Common ; and similarly 
the gravel will be found on the surface of Clapham Common. 
There can be httle doubt, then, that the gravel once spread 
in a continuous sheet over these three commons, as indi- 

From Prof. Pre.stwicli's Ground Beneath Us. 1857. 



I40 PHYSIOGRAPHY. [chap. 

cated by the dotted line, and has been cut through by 
running water. The Wandle is a tributary of the Thames, 
and what is true of this small river is also true of the larger. 
In some places the Thames has cut through similar high- 
level gravels ^ and through the London clay. In other 
parts of the river, again, the chalk forms the two sides of 
the valley through which the water flows. In the case of 
the chalk, it is evident, from what was said in the last 
chapter, that the mechanical erosion would be greatly 
assisted by chemical solution, the carbonate of lime being 
easily soluble ; while the flints, so commonly found em- 
bedded in the chalk, would resist such chemical action 
and, to a great extent, would also withstand mechanical wear. 
Hence their broken fragments will be found still rolling 
about as flint-gravel ; and every bit of such gravel is, in fact, 
a memorial of a quantity of white chalk that has long ago 
been dissolved and washed away by running water. In 
other parts of its course, the Thames flows through rocks of 
a different character, which will be subsequently described ; 
but they are all affected by the mechanical or by the chemical 
action of the river, in the way just explained. 

Passing from the study of the Thames Valley to that 
of the general surface of the country, abundant evidence 
is to be found that rain and running water have been 
actively at work. Indeed there is good reason to believe 
that these almost silent workers have been the chief 
instruments in producing the present physical features 
of the ground. They have eaten out river-courses and 
worn away valleys, leaving masses of rock v/hich stand out 
as hills and crags. But, while giving them credit for effecting 

1 The meaning of the term "high-level gravels" will be explained 
in Cliapter XVII. 



IX.] THE WORK OF RAIN AND RIVERS. 141 

such work as this, it is necessary to recognize the co- 
operation of other forces, the effects of which will be dis- 
cussed in subsequent chapters. 

If running water is thus wasting away the land, year after 
year and age after age, what ultimately becomes of the great 
quantity of matter which must be removed ? To this question 
a partial answer has already been incidentally given. The 
coarser detrital matter is pushed along the bottom of the 
stream, and thus slowly moved towards its mouth ; whilst 
the finer detritus, being held in suspension, is carried more 
rapidly away by the flow of the running water. When the 
flow is checked, the sand and mud settle down, the coarser 
particles being naturally the first to subside. In the minia- 
ture river- systern, self-established in the muddy bank left by 
t.he receding tide, a minute stream may often be seen enter- 
ing a quiet pool of sea-water, and depositing its burden of 
sand, particle by particle, upon the floor of the little pond. 
Exactly the same kind of thing occurs, on a much iarger 
scale, at the mouth of every river. In some cases, a river 
during its course opens out into a lake, and then the 
resemblance to our model on the sea-shore is even more 
striking. On entering the lake, the flow of the stream 
becomes suddenly checked, and a part of the suspended 
sediment falls to the bottom ; so that, by the time the stream 
emerges, its waters have become purified. The eff'ect of 
a sojourn in the lake is somewhat like that of allowing 
muddy water to stand in a glass ; in either case, much of 
the sediment slowly subsides. 

A striking example of the purifying effect of a lake is 
seen in the Lake of Geneva, through which the Rhone 
flows. The river enters the upper end of the lake as a 
turbid stream, laden with detritus brought down from the 
Alps ; but, at the lower end of the lake, it issues forth well 



142 PHYSIOGRAPHY. [chap. 

purged of its impurities. During its passage through the lake, 
the mud which it held in suspension is deposited upon the 
bottom ; and, accordingly, at the entrance of the river, new 
land is being slowly formed by the growth of this sediment. 
In fact, Port Vallais, the Porhis Valesia. of the Romans, 
which was originally situated on the margin of the lake, is 
now nearly two miles inland ; the intervening ground having 
been formed, at the expense of the lake, by accumulated 
sediment deUvered by the river. In this way, a lake may 
grow shallower and smaller, until at length it becomes com- 
pletely silted up ; and a marshy tract is formed, through 
which the river flows in a meandering course. Such land 
is generally called alluvium?- 

It often happens that, without flowing into a lake, a river 
may get relieved of much of its burden of sedimentary 
matter. When an unusual supply of water is suddenly 
delivered into a stream, by heavy rainfall or by rapid thaw 
of snow, the swollen stream rises above its banks, and floods 
the adjacent land. In a flood, or freshet, the water is always 
highly charged with detritus ; and, on the overflow of the 
river, some of this is deposited as a fine layer of mud evenly 
spread over the flooded soil. The overflow being repeated 
season after season, the layers of mud accumulate until they 
form a low alluvial tract on each side of the stream. Most 
rivers are bordered by strips of rich meadow-land, which 
have been formed in this way. Such low-lying alluvial 
meadows are common along the banks of the Thames ; and 
in the lower part of the basin, where the river is broad, 
especially between London and Tilbury, there are great 
expanses of flat marshy ground : the Isle of Dogs, for 
example, is such an alluvial tract. Periodical deposition of 

^ Alluvium, from the Lat. ad, and luo, I wash ; land which is 
added to by the wash or flow of water. 



IX.] THE WORK OF RAIN AND RIVERS. 143 

sediment by means of river-floods is. however, best illus- 
trated by reference to the overflowing of the Nile. After 
the rainy season and the melting of the Abyssinian snows 
in the southern part of the basin of the river, a flood of 
water, charged with detritus, passes down the single channel 
which traverses Egypt, and, overflowing the banks of the river 
m its lower course, deposits the rich alluvial mud to whigh 
that country owes its fertility. 

When a river approaches the sea, the inclination of its 
basin usually diminishes, its speed is slackened; and, con- 
sequently, it deposits more or less of the matter which it 
holds in suspension. If the sea, near the mouth of the river, 
is not much disturbed by currents, as in a protected bay, the 
sediment will accumulate, and form a tract of alluvial land 
which is generally fan-shaped. In Lower Egypt, the Nile 
has, in this way, produced an enormous alluvial tract, which 
was called by the Greeks the Delta, in allusion to its shape 
resembling that of their letter A. About 120 miles above 
its mouth, the Nile divides into two main streams, of which 
the western is known as the Rosetta branch, and the eastern 
as the Damietta branch, these names referring to two towns 
situated at their respective mouths. The two streams in- 
close, with the Mediterranean sea on the north, a triangular 
tract of alluvial land, intersected by a network of channels. 
The apex of this triangle, forming what is called the head 
of the delta, is situated about 25 miles below Cairo. Fig- 
35 shows the form of the Nilotic delta. 

From being originally applied to the triangular land 
about the mouths of the Nile, the term "delta" has come 
into general use, and is now extended to all similar alluvial 
deposits. Even the land which has been described as 
formed in the Lake of Geneva by deposits from the Rhone 
may be called a lacustrine delta. If the silt is thrown down 



144 



PHYSIOGRAPHY. 



[chap. 



under tranquil conditions, on a tolerably level bed, it will 
fall in nearly horizontal layers, regularly spread one upon 
another. Could a clean cut be made through the ground 
in such a delta, the cut sides would expose the edges of a 
number of beds or layers, of which the lowest must needs 
be the oldest or earliest formed, and the uppermost the 
youngest or latest formed ; the materials of the delta are, in 
fact, stratified. 




Fig. 35. — Delta of tlie Nile. 

In following a river from its mouth towards its source, it is 
generally found to be continually branching out into smaller 
and smaller streams, not unlike the ramifications of a tree, 
until, at length, it is lost in a multiplicity of little rills. And, 
in tracing a river downwards into its delta, it is found in like 
manner that it divides and subdivides, till at last it is split 
up into a network of channels, and reaches the sea through 



IX.] THE WORK OF RAIN AND RIVERS. 145 

a number of separate openings. The arrangement of 
branches in the delta is therefore similar to that in the 
catchment-basin, but exactly opposite in direction. In the 
catchment-basin all the branches co7iverge to the main 
stream ; in the delta they all diverge from the trunk channel. 
The difference between the catchment-basin and the' delta 
is shown in Fig. 2)^. 

In many deltas, the alluvial land is swampy, or washed by 
the sea at high tide ; and the alluvium may, in some cases, be 
traced beneath the level of the sea, in the form of shoals 
and sand-banks, which are made up of the lighter particles 




Fig. 36. — Catchment-basin and delta of a river. 

of detritus swept out beyond the true delta. The great 
Indian rivers, the Ganges and Brahmaputra, form together 
a vast delta, consisting, for the most part, of marshy land 
supporting a growth of mangroves and nipa palms. The 
delta of the Mississippi (Fig. 37) is an enormous tract ot 
swampy ground in the Gulf of Mexico, furrowed by 
numerous streams and lakes. Holland may be regarded as 
an old delta, formed by the Rhine and the other rivers that 
pass through it. And, on the coast of this country, we 
frequently find tracts of alluvial land, such as that forming 
Roinney Marsh. Occasionally, the estuaries of our rivers 
become silted up, more or less completely, and thus impede 
navigation. In the times of the Romans, the Isle of Thanet 

I, 



r46 



PHYSIOGRAPHY. 



[chap. 



was separated from the Kentish coast by a channel suffi- 
ciently wide to admit the Roman fleet ; but this channel is 
now choked up, and the so-called island is united by an allu- 
vial tract with the mainland. But, as a rule the rivers of this 
country are not large enough, and have, comparatively, too 




Fig. 37. — Delta of the Mississippi. 



rapid a fall, to produce deltas. Moreover, in tidal rivers, the 
regular to-and-fro motion of the water in its estuary hinders 
deposition of sediment. The scour of the ebb-tide co- 
operates with the rapid flow of the river to sweep away any 
sediment thrown down during the flood-tide, when the 



IX.] THE WORK OF RAIN AND RIVERS. ' 14-, 

downward current of the river is arrested. In some estuariet 
the tidal current is so charged with muddy matter that it is 
artificially carried over low land, in order to cover it with a 
fine silt, called luarp ; this is done in the estuary of the 
Humber. In cases where an actual delta is not formed, a 
bar, or shoal, may be thrown across the mouth of the river, 
and thus interfere with navigation. A river exposed to full 
tidal action, like the Thames, has little chance to form a 
delta; and, although alluvial deposits are to be found on its 
banks, and shoals in parts of its estuary (Fig. 48), there is 
sufficient scouring out of the mouth to keep its channel 
open. 

But, although the Thames forms no delta, the quantity of 
detritus which it carries down from the surface of its basin 
and discharges into the sea is far from being insignificant. 
The proportion of solid matter suspended in water varies 
considerably in different rivers ; and, in the same river, at 
different seasons. Thus, Bischof, in examining the Rhine, 
found that, when the river was turbid, it contained ^sV^ of 
its weight of solid matter in suspension ; but, at another 
season, when the water was clear and blue, it contained only 
g7g-g- Q th part. The Ganges, which has formed such an 
enormous delta, is said to hold, on a yearly average, as much 
as sxoth by weight of suspended detritus. No river has 
been more carefully examined than the Mississippi, and it 
has been determined that the average proportion of sediment 
in this great stream is ysVo ^7 weight, or -g 9V(J ^7 volume ; 
so that the weight of mud actually carried to sea, in the 
course of a year, reaches the enormous amount of 
812,500,000,000 pounds. 

With regard to the Thames, it has oeen estimated that it 
discharges annually 1,865,903 cubic feet of sediment 
(Geikie). Add to this the quantity of mineral matter 

L 2 



r48 PHYSIOGRAPHY. [chap. 

washed away in solution, to which reference was made in 
the last chapter, and it will be found that the total quantity 
of solid matter carried to sea by the Thames is really 
enormous. At Kingston, as has been stated at p. 126, the 
dissolved matter is estimated at about 548,230 tons per 
year. Now reckoning 15 cubic feet to the ton, which is 
about the average weight of chalk, this weight is equivalent 
to upwards of 8 million cubic feet. But this is only at Kings- 
ton, and it is certain that much more is dissolved before the 
river reaches the sea. Nor must we forget to add some- 
thing considerable to represent the quantity of coarse sedi- 
ment pushed along the bed of the river. On the whole, 
then, we shall probably not be far wrong in saying that 
the Thames carries down to the sea, every year, 14 million 
cubic feet of solid matter. 

Imagine a huge die-shaped mass of stone measuring 100 
feet in length, 100 feet in width, and 100 feet in height : 
this would contain one million cubic feet. No fewer, then, 
than 14 of these gigantic cubes appear to be quietly stolen 
from the surface of the Thames basin by means of running 
water, and transported to the sea, in the course of a single 
year. But the Thames basin covers a very large area, and 
it will be found on calculadon that, admitting the abstraction 
of this vast mass, the entire surface of the basin would be 
reduced in level by only g^th part of an inch every year. 
At the present rate of wear and tear, therefore, denudation 
can have lowered the surface of the I'hames basin by hardly 
more than an inch since the Norman conquest ; and nearly 
a million years must elapse before the whole basin of the 
Thames will be worn down to the sea-level. This method 
of showing the amount of work effected by rain and rivers 
in wearing away the land was suggested by Mr. A. Tylor, 
and has since been applied with interesting results by other 



ix.j THE WORK OF RAIN AND RIVERS. 



149 



geologists. Thus, Prof. Geikie has calculated' that, at the 
present rate of denudation, it would require about 5^ 
million years to reduce the British Isles to a flat plane at 
the level of the sea. It must be remembered, however, 
that such calculations are beset with grave difficulties, and 
that the results can only be put forth as rough approxima- 
tions. Nevertheless, they are not without their value in 
enabling us to form a conception of the great wearing down 
of land which must be effected by rain and rivers. 

1 "On Modern Denudation." By Archibald Geikie, F.R.S. Trans 
actions of the Geolo^cal Society of Glasgow, vol. iii. p. 153. 



CHAPTER X. ^ 

ICE AND ITS WORK 

Although our attention was restricted in the last chapter 
to the action of rain and rivers, it would be a great mistake 
to suppose that these are the only agents by which denu- 
dation is effected. Rain and rivers unquestionably do much 
in the way of destruction, but they work with far greater 
effect when aided by the action of frost. Bare faces of 
hard rock may be exposed to the action of rain, year after 
year, without suffering any marked change : the water may 
fill the pores and fissures of the rock ; yet, unless the mineral 
components happen to be easily decomposed, it will eat 
its way into the stone with extreme slowness. But when a 
frost comes on, the conditions are entirely changed, and a 
fresh element of destruction is introduced. The water 
with which the rock is charged freezes into ice and, during 
its solidification, it tends to expand, as explained in a 
previous chapter. If the water be confined in the pores 
and cracks of the rock, the tendency is resisted ; but the 
particles, in freezing, push each other apart in all directions, 
with such force, that the strongest rock is sooner or later com- 
pelled to yield. Just as a water-pipe bursts during a frost, 
SO the rock ultimately gives way. Fragments of stone, often 



CH. X.] ICE AND ITS WORK. . 15) 

of large size, are thus rent from the rock and ready to 
tumble down at the next thaw, exactly as the flakes peel off 
a stuccoed wall after a hard frost. Nor should it be for 
gotten that frost does excellent work for the farmer in 
breaking up hard ground. A stiff soil is more or less 
loosened after a thaw, and is thus brought easily within the 
reach of other denuding agents. 

In addition to the mechanical force exerted by water 
during freezing, there are other ways in which ice assists in 
the destruction of the land. In a country with a mild 
chmate, like that of Britain, the effects of ice are extremely 
feeble ; yet they are not altogether wanting, even within the 
basin of the Thames. It has been explained in Chapter IV., 
that when a body of water is cooled, it shrinks in bulk, like 
other substances ; but it shrinks only when cooled down to 
a certain temperature. In fact when water is reduced to 
about 39° Fahr. (4° Cent.) ^ its molecules are packed more 
closely together than at any other temperature, so that 
whether you raise or lower the temperature above or below 
this point, precisely the same effect is produced ; the bulk 
of the liquid is increased. At 39° Fahr., therefore, water 

1 The thermometer commonly used in this country is graduated 
according to a plan introduced by Daniel Gabriel Fahrenheit, a native 
of Dantzic, who settled at Amsterdam in the beginning of the last 
century, and became famous as a thermometer maker. In Fahrenheit's 
instrument the distance between the freezing and boihng points of water 
is divided into iSo equal parts, or degrees, and the zero or starting- 
point of the scale is arbitrarily placed 32 degrees below this freezing 
point. On the Continent another scale is commonly used, known as 
the centigrade scale, since the distance between the freezing and boiling 
points of water is divided into one hundred degrees. The centigrade 
scale is now frequently used in scientific investigations in this country. 
As a given temperature is indicated by different numbers on the two 
scales, they are distinguished by addition of "Fahr." and " Cent." to 
the readings or simply by the initials F. and C. 



152 



PHYSIOGRAPHY. 



[CHAP 



is said to have its maximum density. This can easily be 
observed by repeating an old experiment devised originally 
by Dr. Hope. Insert two thermometers (Fig. 38), at 
different levels into a cylinder of water, and chill the water 
by applying ice around the middle of the vessel. As the 
water becomes cooled it grows denser, and therefore sinks 
to the bottom, so that the lower thermometer falls until it 
reaches 39° Fahr. Further cooling then expands the water, 
instead of condensing it, and consequently the cold water 
rises, so that now the upper thermometer, which has mean- 
while been almost stationary, 
begins to fall, and continues 
falling until, like the lower one, 
it reaches 39° Fahr. The whole 
body of water is then at its maxi- 
mum density, and any further 
reduction of temperature causes 
expansion, the cold water becom- 
ing specifically lighter and rising 
to the surface. Gradually, the 
upper thermometer sinks to the 
freezing-point, and then a layer 
of ice begins to form upon the 
surface. This experiment roughly 
imitates what occurs in a natural piece of water, such as 
a lake : the surface freezes, while the bottom-water remains 
several degrees warmer. 

At the moment of freezing, when the particles of water 
are trying to arrange themselves in those crystalline forms 
which were noticed in Chapter IV., there is an increase 
of bulk mucli greater than that just described. 

Ice, being thus relatively much lighter than water, floats 
upon the surface. Yet there are certain conditions under 




Fig. 38. — Hope's experiment on 
the contraction of water. 



X.J ICE AND ITS WORK. 153 

which ice may be actually formed at the bottom of a 
stream, and remain there for some time. This forma- 
tion of ground-ice is occasionally seen in parts of the 
Thames, 

Dr. Plot, the first keeper of the Ashmolean Museum at 
Oxford, published in the year 1677 a famous work on the 
Natural History of Oxfordshire, in which he refers to the 
freezing of the Thames in the following words : — " I find 
it the joint agreement of all the watermen hereabout that 
I have yet talked with that the congelation of our river is 
always begun at the bottom, which, however surprising it 
may seem to the reader, is neither unintelligible nor ridi- 
culous. They all consent that they frequently meet the ice 
meers (for so they call the cakes of ice thus coming from 
the bottom) in their very rise, and sometimes in the under- 
side including stones and gravel." 

To explain the formation of such ground-ice, it has been 
suggested that the action of the running stream mechanically 
mixes the cold surface-water with the warmer water below 
until the temperature becomes uniform throughout ; and 
when the air is very cold the whole mass may thus be 
reduced to the freezing-point. The formation of ice will 
then be determined at the bottom, in consequence of the 
greater tranquillity of the water and the contact of cold 
stones and other objects which have become chilled by free 
radiation. This ground-ice is generally found in little masses 
clinging to stones and weeds ; and, when the temperature 
rises after sunrise, the loose bodies are lifted to the surface by 
the ice, just as if buoyed up with corks. The ice then floats 
down the river, bearing its little freight of gravel, which is 
dropped on the bed of the river when the ice is broken up 
or melted. The Rev. J. C. Clutterbuck, who has paid 
great attention to the study of the Thames, tells us that he 



154 PHYSIOGRAPHY. [chap 

has seen "pieces of rock, eight pounds in weight, raised by a 
mass from the bottom and carried down the river." ' Here 
then is a geological agent not to be neglected, since it 
assists the transporting power of streams in carrying solid 
matter from the land seawards. But, if the geological im- 
portance of ice is to be fully realized, attention must be 
turned from these trivial illustrations to the grand spectacles 
which are presented by masses of moving ice in mountain 
regions beyond the limits of our own islands. 

When a snow-storm occurs in this country, the snow does 
little or nothing in the way of denudation, beyond what it 
may effect indirectly, by giving rise to floods when a rapid 
thaw takes place. In fact, the snow, as snow, protects 
rather than destroys. But the result is different in a 
mountainous country, such as that of the Swiss Alps. The 
greater part of the snow which falls there above the snow-line, 
as explained in Chapter IV., lies all the year round un- 
melted ; and, therefore, every fall must needs add to the 
thickness of the heap piled upon the mountain-top. It is 
true that the snow evaporates, but the evaporation is 
extremely slow, and is far from equal to the additions con- 
stantly received ; and, though the heat of the sun during 
the day, may melt the surface layer, the water thus formed 
sinks in and becomes frozen in the interior of the mass. 
Occasionally, the accumulation is retieved by a great mass 
of snow sliding down the mountain slope, as an avalanche. 
Usuall)'', however, the pressure of the heaped-up snow gets 
rid of the surplus by gently squeezing it into the valleys 
below, where it moves down with extreme slowness. Yet 
it does not come down as a mass of white opaque snow. 
It has been shown in an earlier part of this work (p. 64) 
that snow is white and opaque in consequence of the 
1 Report of the Thames Commissionirs, Appendix i. 1866. 



X.J ICE AND ITS WORK. 155 

air entangled among its crystals. In squeezing a handful 
of snow into a snowball, some of this air is forced out, and 
the loose crystals begin to adhere to one another ; while, by 
compressing snow very tightly in a hydraulic press, it may 
be rendered almost homogeneous and thus brought nearly 
to the condition of ice (see p. 15S). In this way, the great 
pressure exerted by the piles of snow in the Alpine snow- 
fields compresses the lower layers, and converts them more 
or less completely into ice. The imperfectly consolidated 
substance, partly snow and partly ice, is known in Switzer- 
land as Neve or Fii-n. Moreover, the water produced by 
temporary thaw, during sunshine, becomes frozen into ice ; 
and, in these and other ways, the water, which fell on the 
mountain-top as loose white snow, is ultimately sent down 
into the valleys in the form of solid ice. The river of ice 
which thus drains the high snow-fields is termed a glacier 
(Fig. 39).' 

Although we have just spoken of a " river of ice," it is not 
easy, at first, to believe that a substance so solid and rigid can 
really move in any way like a mobile liquid. Yet the fact 
that the glacier does so move can easily be demonstrated. 
Drive a row of stakes firmly into the ice across a glacier and 
opposite to some well-marked point, as at a in Fig. 40, so 
that you may know exactly their position. If you examine 
these stakes a week or two afterwards, you will find that they 
are no longer at a, but at some point lower down the glacier, 
say opposite to b. The ice has therefore moved during this 
time from a to B, carrying the stakes with it. 

From this experiment it is seen that the ice really moves. 
But the experiment teaches something more than this ; for 
it will be observed that the stakes have not only moved 
down, but have changed their relative positions. Instead of 

' Yxfixa. Kgas,i\zi> Etitdis stir lei Glaciers. Neuchatel, 1S4.0, 



'^ 







^"^ , 








Fw. 39- — Glacier of ZertnaiL 



CK. X.] 



ICE AND ITS WORK 



157 



forming a straight line across the ice, as at a, they now form 
a curve at B ; the stakes in the middle of the row have got 
farther from A than those at the sides, and it is therefore 
clear that they must have moved faster. But the movement 
of the stakes is due simply to the movement of the ice, so 
that if the middle stakes move faster than the side ones, it 
shows that the middle of the glacier moves faster than its 
sides. Exactly the same thing 
may be observed in a river : light 
bodies floated on a stream move 
like the stakes carried down by 
the glacier. Nor is it difficult to 
see why a river should flow more 
rapidly in the, middle than at its 
sides. The particles of water at 
the sides rub against the banks, 
and consequently are not so free 
to move as the particles in the 
middle of the stream. In like 
manner, friction against tlie rocky 
walls on the flanks of a glacier 
causes the ice at the sides to 

move more sluggishly than the ice in the middle. Again, 
it is known that, in a river, the particles at the bottom drag 
along the bed and move less rapidly than those at the 
surface. The ice of a glacier behaves in precisely the same 
way. It is concluded, therefore, that the motion of a glacier 
is like the motion of a river. If the glacier enters a gorge, 
It -becomes contracted and the flow is rapid : while, if its 
bed widens, it spreads out and the movement becomes 
slower. In truth, in all points, the motion of a glacier 
resembles that of a river; the movement is essentially 
the same in kind but difTerent in degree, the rate ol 




Fig. 40. — Motion of a glncier. 



IS« PHYSIOGRAPHY. [chap. 

movement of the glacier being perhaps only a few inches 
or, at most, a yard or two, daily. 

This sluggish motion of a glacier, and the way in which it 
accommodates itself to all the inequalities of the surface 
over which it travels, long ago gave rise to the supposition 
that ice is a plastic or viscous substance, something Hke 
dough or even treacle, so that it can sink into a depression, 
or ride over a ridge, without losing its continuity. Yet, as 
a matter of fact, ice is so brittle that if you pull, or try to 
bend, it, it will snap, without stretching to any appreciable 
extent. How, then, can the apparent plasticity be recon- 
ciled with the undoubted brittleness ? Prof Tyndall ' has 
shown the way out of this difficulty. 

When a schoolboy makes a snowball, he squeezes a hand- 
ful or two of light snow into a hard compact lump ; and it is 
worth noting that, if the snow be just on the point of thaw- 
ing, he will be able to weld it into a firmer mass than if he 
employed perfectly hard and dry snow. Snow, as we have 
seen, is nothing but a confused mass of ice-crystals ; and 
the snowball becomes hard, partly, because it contains less 
air, and, partly, because the little pieces of ice of which it 
is composed, instead of remaining loose, stick firmly to one 
another. But why do they thus become welded together? 
Experiment shows that when two pieces of damp ice are 
pressed together, they immediately freeze into one solid 
mass. Faraday observed this curious fact five-and- twenty 
years ago, and the phenomenon has been termed regelation. 
Hence, when snow is strongly squeezed, the icy particles 
freeze together into a compact substance ; and, hence, the 
snow from which a glacier takes its birth, is pressed by the 
weight above into a hard mass, more or less like true ice. 

» See The Glaciers of the Alps, by John TyndaU, F.R.S. i860. 



X.J ICE AND ITS WORK. . 159 

A number of pieces of ice, powerfully scjueezecl logether 
in a hydraulic press, are readily united into a solid lump, 
or a single mass may be crushed, and the fragments built 
up into a differently shaped body. In a similar manner, 
when a glacier is forced over an obstacle, the ice, being 
brittle, cracks and snaps, but the enormous pressure of 
the sliding mass behind, squeezes it together again, and 
regelation immediately heals the fractures. The glacier 
therefore accommodates itself to irregularities in its bed, 
not by virtue of any real plasticity, but by being succes- 
sively fractured and frozen. In fact, by suitable means, ice 
may be artificially moulded at will, as though it possessed 
true plasticity ; and a similar operation is doubtless carried 
on in nature. 

As it creeps down the valley, the glacier transports, from 
higher to lower levels, any detrital matter that may happen 
to fall upon its surface. From th'e neighbouring rocks, 
fragments are constantly being loosened by atmospheric 
agents, and these, sooner or later, tumble down upon the 
glacier. In this way, a line of debris fringes each side of 
the glacier, some of the stones being perhaps several tons in 
weight. Such accumulations of detritus are known as f;io- 
raines ; and, as those which are now being described, occur 
on the two sides of the ice-river they are distinguished as 
lateral moraines. As the glacier moves along, the moraine- 
matter is carried forwards, until, at length, it reaches the end 
of the glacier ; and thus fragments of rock may be transported 
down the valley, far from the heights above. The water 
which issues from the melting ice, at the end of the glacier, 
is unable to carry off this burden of stones which the ice has 
deposited ; and, hence, we generally find, across the end of 
the glacier, a confused heap of rubbish, known as a ten/iinal 
^lorame.. When two streams of ice unite, the lateral 



i6o 



PHYSIOGRAPHY. 



fCHAP. 



moraines unite also, as in Fig. 41, where a B c d represent 
the four lateral moraines of two glaciers. It is clear that, 
after the union of the two branch-streams, the outer moraines, 
A and D, will continue to occupy the sides of the trunk- 
glacier; while the two inner moraines will unite at the fork 
E, and form only one ridge of detritus, which will be carried 
along the middle of the main glacier. This middle line of 
stones is therefore distinguished as a inedial moraine. If a 




Fig. 41. — Lateral and medial moraines. 



glacier receives in its course many side-branches, each con- 
tributing its moraines, the entire surface of the ice may 
ultimately become strewn with rubbish. 

A glacier resembles a river, not only in its power of thus 
transporting detritus from a higher to a lower level, but also 
in acting as a direct agent of denudation. Just as a river 
wears away its banks and its bed, so the ice acts on the sides 
and bottom of the valley along which it travels. If the 



X.] ICE AND ITS WORK. i6i 

ice has to turn a sharp corner, or make an abmpt descent, it 
is forced to split, and in this way yawning chasms, perhaps 
hundreds of feet in depth, are produced in the glacier. 
Such rents are termed crevasses. Stones, sometimes of great 
size, fall with a crash down these clefts, and reaching the 
bottom of the glacier get frozen into its base. As the 
glacier moves, these stones, pressed by the weight of ice 
above, scratch and score the rocky bed in the direction of 
the ice-flow; while the stones themselves, jammed in between 
the ice and its floor, get bruised in turn, so that by the time 
they are discharged at the terminal moraine they may be 
covered with parallel scratches. 

At the same time, the smaller fragments worn off the rocks 
by the passage of a glacier get ground down into fine 
gravel, sand, and mud, which may be carried in suspension 
by the stream of water which flows over the bed of the 
glacier. For it should be noted that the bottom layer of 
ice, pressed by the weight above, and grinding along the 
floor, is generally in a state of thaw ; and, moreover, water 
finds its way from the surface to the bottom through cre- 
vasses. Hence, a little liquid stream separates the bottom ice 
from the rocky bed; and at the end, or snout, of the glacier 
this water issues forth, not indeed as a clear bright spring, but 
as a thick stream laden with detritus. The Rhine, the Rhone, 
the Po, the Ganges, and many other large rivers, may be traced 
back to muddy streams springing from glaciers. The fine 
detrital matter which the water thus carries along polishes 
the surface of the rock over which it flows. The action of 
a glacier is consequently twofold : the fine sandy matter 
polishes the surface, while the large stones scratch furrows. 
It is, in fact, as though some giant hand had rubbed the 
surface of the rock with fine emery powder, and at the san.e 
time rasped it with a huge file. 



l62 



PHYSIOGRAPHY. 



[chap. 



All rough points of rock in the path of a glacier are thus 
rubbed down, and projecting masses are smoothed to the 
form of rounded bosses. The flat-domed hummocks of 
rock produced in this way are termed sheep-backs or roches 
vioiitotinees (Fig. 42), since, if seen in the distance, tliey bear 







f% ^^ 




Fin. 42. — Roches I\Ioutonnees Creek, Colorado (Hayden). 



some resemblance to a flock of sheep. Hence, the passage 
of a glacier across a country gives rise to peculiar features 
not produced by any other agent of denudation ; and, by 
these peculiarities, we may tell, with certainty, that ice has 
been at v/ork in a district where there is, perhaps, not a 
vestige of ice at the present day. Thus, in many of the 



X.] ICE AND ITS WORK. • i6^ 

valleys of Switzerland, not now occupied by glaciers, the 
rocks are rounded, polished, and scratched, showing that 
the Swiss glaciers must formerly have been of gigantic 
proportions and that they extended far beyond the limits 
retained by their present successors. 

On travelling northwards, the snow-line is found to 
descend until, in the Arctic Regions, it comes down to the 
very sea-level. Hence, in such regions, the entire surface of 
land may be enveloped in a mantle of ice. This ice-sheet 
creeps down towards the shore, until its foot at length 
advances into the sea. Huge masses of ice then become 
detached, and are sent drifting away as icebergs. These 
mountains of ice often assume most fantastic shapes ; and 
their vast mass produces so great a depression of tempera- 
ture in the neighbouring air that, when they are carried into 
the Atlantic, they are usually obscured by a shroud of mist 
The icebergs, like glaciers, are laden with fragments of rock 
worn from the land over which the ice-sheet travelled ; and 
when, on reaching warmer waters, they melt, they discharge 
this freight of stones and earth, which may thus get carded 
far from their original home. When blocks of rock are 
borne along by running water, they become rounded by the 
friction to which they are subjected ; but, when a fragment 
of rock is transported on an iceberg, it may retain much of 
its angularity and be dropped upon the sea-bed in an 
almost unworn state. The finer detritus which the berg 
carries will be diffused through the water in which the ice 
melts ; and currents may transport it far away into southern 
latitudes. If a glacier descends to the edge of a lake, exactly 
the same thing occurs as in the formation of an iceberg. 
A tongue of ice is pushed into the water, and bergs break 
off and float away, carrying their burden of moraine matter 
to be strewn over the bottom of the lake on the melting of 

M 2 



i64 PHYSIOGRAPHY. - lCHAp, 

the floating ice. If the bottom of the lake, or of the sea, 
should at any time be upheaved, the glacial mud and gravel, 
with angular blocks and ice-scratched boulders, may be 
exposed to view ; and may thus furnish evidence of glacial 
denudation in countries which are now free from anything 
like glaciers or icebergs. 

Other evidence of ice-action is afforded by the peculiar 
position of large angular blocks of stone, poised perhaps 
upon the very edge of a precipice, or balanced upon a mere 
point. Such stones, known 2l.% perched Mocks or blocs perches, 
could hardly have been brought into their strange position 
by mere rolling, or by the action of running water : but it is 
easy to see that they might have been dropped by an ice- 
berg, or left stranded by the gradual melting of a glacier on 
which they were originally seated. 

It is now more than a quarter of a century since the late 
Prof. Agassiz, who had been a diligent observer of glacial 
action in Switzerland, visited this country, and in company 
with Dr. Buckland pointed out the evidence of former ice- 
action in many parts of Britain. The traveller in Scotland, 
Ireland, Cumberland, or North Wales, will have no diffi- 
culty in detecting roches montonnees, perched blocks, and 
occasionally the remains of old moraines ; while here and 
tliere, where the rocks have been protected, the glacial 
polish and striations are still preserved. Such evidence 
conclusively proves that ice must have flowed over the sur- 
face of the country. It is believed, indeed, that at one 
period of geological history, known generally as the Glacial 
Period, Britain must have been buried beneath a vast sheet 
of ice, similar to that which now covers Greenland. This ice 
played its part in rasping and grinding and polishing the 
surface of the land ; and it has even been suggested by Prof. 
Ramsay that many of the rock-basins which contain lakes 



X.] ICE AND ITS WORK. ■ 165 

have been scooped out by the action of huge masses of 
moving ice. Nor is it only the effects of land-ice which 
the glacialist sees marked upon the rocks of Britain. During 
part of the Glacial Period, the land must have been sub- 
merged beneath the waters of an icy sea ; and icebergs, 
drifting from the north, scattered their freight upon the 
rocky floor which has since been upheaved as dry land. 
Even in the very neighbourhood of London, as at Finchley, 
deposits of gravel and clay may be found crowded with ice- 
borne boulders, which still retain their glacial scratches. 
This drift, as it is termed, not to cite other evidence, suffi- 
ciently shows that there must have been a time when ice 
played its part, as an agent of denudation, vt'ithin the basin 
of the Thames. 



CHAPTER XI. 



THE SEA AND ITS WORK. 



At Margate, where the estuary of the Thames ends in 
the North Sea, even a bhnd man could not stand long upon 
the shingly beach without knowing that the sea was busily 
at work. Every wave that rolls in from the open ocean 
hurls the pebbles up the slope of the beach ; and then, as 
soon as the wave has broken and the water has dispersed, 
these pebbles come rattling down with the currents that 
sweep back to sea. The chattel of the beach thus tells us 
plainly that, as the stones are being dragged up and down, 
they are constantly knocked against each other ; and, it is 
evident, that, by such rough usage, all angular fragments of 
rock will soon have their corners rounded off, and become 
rubbed into the form of pebbles. As these pebbles are 
rolled to and fro upon the beach they get worn smaller and 
smaller, until, at length, they are reduced to the state of sand. 
Although this sand is at first coarse, it gradually becomes 
finer and finer, as surely as though it were ground in a mill ; 
and, ultimately, it is carried out to sea as fine sediment, and 
laid down upon the ocean-floor. 

On examination of the chalk cliffs, which back the beach, 
it is easy to see how these suffer by the constant dash of 



CH. XI.] THE SEA AND ITS WORK. . i6? 

the waves. Rain, frost, and other atmospheric agents, 
playing their part in the work of destruction, attack the chft 
and dislodge masses of rock which come tumbling down to 
its base, where they accumulate as a line of rubbish. As 
soon as the fragments are brought within reach of the waves, 
they are rolled against the clififj bruising and battering the 
face of the rock, while the fragments themselves are apt to 
get shivered in the fray. 

During violent gales the breakers acquire unusual power, 
and are able to move rocks of enormous weight. On the 
western coast of Britain, where the Atlantic breakers roll in 
upon the shore, they have been known to exert a pressure 
of between three and four tons on every square foot of 
surface exposed to their fury. Even in summer, these waves 
break upon the coast with a pressure of about 600 pounds 
per square foot ; and, in winter, this force is often trebled. 
It is easy to believe that such masses of moving water can 
carry with them huge blocks of stone, and hurling these 
against the shore, can breach it just as effectually as though 
it were attacked by the blows of a battering-ram. In fact, 
whether in storm or in calm, a cannonade, more or less 
sharp, is constantly kept up against the coast, the ammuni- 
tion being supplied by the ruins of the coast itself 

Were the waves to break upon the shore without the aid 
of any fragments of rock, the mere weight of water would 
naturally effect some amount of destruction ; but, there is 
reason to believe that, in most cases, this would be 
comparatively slight. It has been already shown that a 
river erodes its channel, not so much by its own friction, as 
by that of the sedimentary matter which it sweeps along in 
its course. In like manner, the wear and tear of the waves 
themselves is insignificant compared with that wrought by 
the boulders and pebbles, the gravel and sand, which they 



i68 PHYSIOGRAPHY. [chap. 

bring to bear upon the coast. Every wave carries, as it 
were, a number of stone hammers, with which it bruises and 
batters the clififs ; and, as this action is persistently repeated 
by wave after wave, the hardest rock is at length forced to 
yield. 

Almost any part of our coast-line will serve to show the 
destructive effects of the sea. It is true, the action is much 
less marked in some directions than in others ; while, at 
certain points, the sea may be engaged, not in destroying, 
but in actually forming land, by deposition of sedimentary 
matter resulting from the destruction of the shore elsewhere. 
As a rule, however, abundant evidence of marine waste may 
be seen on any visit to the seaside. Bays and coves may 
be hollowed out in one part of the coast, and a headland 
may be worn away in another : here, caves are being ex- 
cavated in the base of a cliff; there, tunnels are drilled 
chrough some projecting rock; while, in many places, wall- 
]ike masses are partially detached from the cliffs so as to 
stand out as buttresses, or are even completely isolated in 
the form of " needles," " stacks," and "skerries." A good 
example of marine denudation is furnished by the well- 
known Needles off the Isle of Wight (Fig. 43). A ridge of 
chalk runs across the island from east to west, and it is 
evident that the outstanding wedge-shaped masses were 
once connected with this main body, though now completely 
surrounded by the sea. The headlands of chalk have been 
beaten about by the waves until a passage has been forced 
at a weak point, here and there; and pillars of chalk have 
thus been separated from the mainland. 

Where cliffs are forir.ed partly of hard, and partly of soft, 
rocks, the latter will naturally be more easily attacked by 
the waves. The fantastic forms which sea-cliffs assume may 
often be explained on this principle ; the harder beds, or 



XI.J 



THE SEA AND ITS WORK. 



169 



dykes, of rock standing out in bold relief when the neigh- 
bouring softer rocks have been eaten away. The oldest, 
and, as a rule, the hardest rocks of Britain are developed in 
the western and northern parts of the island, and hence the 
sea acts with less effect upon them than upon the softer 
rocks in the east and south of England. Even cursory- 
inspection of a map of England and Wales serves to show 
how the flowing outlines of the chalk coasts of Norfolk, 




-The Needles, Isle of Wight. 



Lincolnshire, and Yorkshire, contrast with the sharp outlines 
and bold headlands formed by the old rocks of western 
Cornwall, Pembrokeshire, and Carnarvonshire. 

In the estuary of the Thames, the rocks are comparatively 
soft, consisting for the most part of sands, clays and chalk. 
Within the Thames Basin, then, there should be no difficulty 
in obtaining evidence of marine waste. Thus Sir C. Lyell 



!7o PHYSIOGRAPHY. [chap 

has pointed out that the Isle of Sheppey has suffered con- 
siderably by the inroads of the sea, fifty acres of land 
having been lost within the short space of twenty years, 
though the cliffs there are from sixty to eighty feet in height. 
Heme Bay, on the Kentish coast, has lost land to such an 
extent that it no longer retains its shape as a bay. Going 
yet further out into the estuary of the Thames, we find a 
notable illustration of marine destruction at Reculver. 
This was the old Roman station of Regulbiwn. Not only 
has the sea entirely destroyed the militar}^ wall, but the 
church, which in the time of Henry VHI. was nearly a mile 
inland, is now on the very brink of the cliff; and, indeed, it 
has only been saved from actual destruction by artificial 
means. As the two towers of the church form a well-known 
landmark to mariners, a causeway has been constructed on 
the beach to arrest the progress of the sea. 

If the sea were a bod}' of water in perfect repose, it 
would be utterly incapable of effecting mechanical erosion. 
But every one knows that the sea is never absolutely at rest, 
and that, even in the calmest weather, its surface is ordi- 
narily more or less troubled with waves. It is easy to 
understand how these are formed. When you blow upon 
the surface of a basin of water, the mechanical disturb- 
ance of the air is imniediately imparted to the liquid, 
and the surface is thrown into a succession of ripples. 
In like manner, every disturbance of the atmosphere finds 
its reflex on the surface of the natural waters. Each puff 
of wind catches hold of the water, and heaps it up into 
a little hill with the face to leeward ; then the crest falls, 
and the water sinks down into a trough, as deep below 
the mean surface as the hill was high above it ; but the 
next column of water is then forced up, only however to 
be pulled down again, and in this way the motion of the 



XI.] THE SEA AND ITS WORK. - I71 

wave may be propagated across a broad expanse of water. 
Drop a stone into a pond, and the same kind of action 
will be seen : the water all round the spot where the stone 
falls is first depressed in a little cup, and then rises again, 
the motion being taken up by the neighbouring water ; and 
a succession of circles, each wider than the last, spreads 
over the pond, until the ripples at length die away upon 
the shore. If any light object, such as a cork, happens 
to be floating on the surface, it will serve to indicate the 
the motion of the water below. As the waves reach it, the 
cork rises and falls, but it is not carried forward by the 
movement of the water. Exactly the same kind of action 
may be witnessed at sea. If a gull, for example, is seated on 
a wave it is simply rocked up and down, and not moved 
onwards. 

Such simple observations are suflicient to show that the 
motion of the water is a movement of undulation and not 
of translation ; it is merely the form of the wave, and not 
the actual water, that travels. The motion is transmitted 
from particle to particle, to a great distance ; but the par- 
ticles themselves perform very small excursions, merely 
vibrating up and down, or rather revolving in vertical 
circular paths. The general effect is similar, as has often 
been pointed out, to that witnessed when a gust of wind 
sweeps across a field of corn. Notwithstanding the im- 
pression produced on the observer, he knows that any 
movement of translation is here quite out of the question ; 
the stalks are not uprooted and carried across the field, 
but each stalk simply bends down before the wind and 
then returns to its erect position. Similarly, in the open sea, 
the wave, or pulsation, is propagated, but the mass of the 
water at any given spot remains stationary, except in so far 
as it vibrates up and down. The mechanical force of the 



172 PHYSIOGRAPHY. [chap. 

wind, however, urges the surface-water forwards to a small 
extent. A fresh breeze tears off the water from the crest 
of a wave, and scatters it as spray, and a heavy gale con- 
verts this into blinding showers of salt rain. The wind too 
catches the top of the wave, and causing it to move faster 
than the water below, urges it to leeward in the form of a 
graceful curl, the edge of which breaks into foam. On 
reaching a shore, the retardation of the deeper part of the 
wave by friction against the sea bottom, increases the 
relative velocity of the superficial part, and the latter 
rolls over; the water bursts with great force upon the 
land, and then sweeps back, as a powerful " undertow," 
to the sea. 

However agitated the surface of the sea may be, there is 
reason to beHeve that the disturbance never extends far 
downwards. The more violent the wind, the greater of 
course will be the agitation which it is capable of pro- 
ducing ; but, even during a storm, the waves never attain 
to anything like the height which is often popularly ascribed 
to them. It is not uncommon to hear of the sea running 
"mountains high;" yet, in a strong gale in the open ocean 
the height of a wave, from crest to trough, rarely exceeds 
forty feet. In the shallow seas around our own islands, 
they are far from attaining to such a magnitude ; the largest 
waves, even in a storm, not exceeding eight or ten feet in 
height. The disturbance produced by such waves extends 
downwards to only a comparatively small depth. In fact, 
the motion of the largest waves is almost imperceptible at a 
depth of about 300 fathoms, or 1,800 feet ; while the agita- 
tion produced by ordinary waves must be quite insignificant 
at one-third of this depth. So far, then, as the destruction 
of the land by the sea depends on the mechanical action 
of such waves, it must cease at about 100 fathoms. Indeed 



XI.] THE SEA AND ITS WORK, • 173 

il is probably very feeble at depths much less than this ; 
and, in most cases, on our own shores, it is not very marked 
below the limit of the lowest tide. 

Winds not only agitate the sea and produce irregular 
waves, but where they are constantly blowing over the ocean 
in a definite direction they cause the surface-water to take 
a similar course, and thus produce steady drifts or currents. 
Dr. CroU has shown that the direction of the great ocean 
currents agrees very closely with that of the prevailing 
winds. Bottles thrown overboard from ships in the open 
ocean may be carried by these currents for hundreds of 
miles, and ultimately cast upon distant shores. Pieces of 
wood, and nuts and seeds, known to be native to the West 
Indies and tropical America, are occasionally drifted across 
the Atlantic, and are washed on to the western shores of 
England, Scotland and Ireland, and even across to Norway. 
In like manner, the Portuguese men-of-war {Fhysalia, Velella) 
and those oceanic snails with violet shells called lanthmcs, 
are now and then brought as visitors to our coasts, though 
usually confined to warmer seas far to the south and 
west. 

Perhaps the best known of these oceanic currents is the 
Gulf Stream, which is a broad body of warm water sweep- 
ing out of the Gulf of Mexico, through the Strait of 
Florida. After running northwards, nearly parallel to the 
coast of the United States, it strikes across the Atlantic 
Ocean in a north-easterly direction. Warm currents, which 
continue the direction of the Gulf Stream, set on to the 
western shores of Britain and even extend to the coast of 
Norway ; while, other currents, parting with these in mid- 
ocean, turn to the south and sweep round the coasts of Spain 
and Northern Africa. The cause of the Gulf Stream is un- 
doubtedly to be sought in the so-called " Trade Winds," whicli,, 



174 



PHYSIOGRAPHY. 



[chap. 



constantly blowing more or less from the north-eastward, 
give a westerly impulse to the inter-tropical surface waters of 
the Atlantic, and thus create the current, which sets into the 
Gulf of Mexico. But, whether the stream, after it leaves 
the coasts of the United States, retains sufficient impetus to 
carry it to our shores ; or whether, as some believe, the true 
Gulf Stream is lost in the middle of the Atlantic, and any 




Ftg. 44. — Map of the Atlantic, showing course of the Gulf Stream. 



warm currents felt on our own coasts are due to the pre- 
dominant south-westerly winds of the temperate part of the 
Atlantic, is as yet uncertain. 

The general course of the Gulf Stream is shown in Fig. 
44. Where the water issues from the Gulf of Mexico, through 
the Florida Narrows, it has a temperature of upwards of 80° 
Fahr. and moves at the rate of between four and five miles 
an hour. In passing across the Atlantic, the current widens. 



XI.J 



THE SEA AND ITS WORIC 



175 



and its speed L= slackened, but it cools with extreme slow- 
ness, so that it carries along a considerable store of heat. 



SANDY HOOK 




Fig. 45.— Section of the Atlantic Ocean between Sandy Hook (near New York) and 
Bermuda, a distance of 700 nautical miles. The figures above the dotted lines 
indicate depths in fathoms. Drawn to true scale this diagram would be about 
five feet wide. 

The stream forms, in fact, a sharply-defined river of warm 
water f owing over the colder water of the ocean. 



176 PHYSIOGRAPHY. [chap. 

That the Gulf Stream is an extremely shallow body of 
water is well seen in Fig. 45, which is reduced from one of 
Sir G. Nares' reports to the Admiralty on the Challenger 
expedition. It represents a section of the North Atlantic, 
between New York and Bermuda ; and it shows in a very 
striking manner, that, when compared with the great depth 
of the ocean, the Gulf Stream is extremely superficial. It 
may indeed be regarded as a mere rill of warm water 
running over the surface of the sea;, for, while the water 
below is considerably more than 2,000 fathoms in depth, 
the Gulf Stream itself is not more than 100 fathoms deep. 
It is seen, too, that, while the Gulf Stream has here a tem 
perature of 75°, the bottom water has as low a temperature 
^s 35°3 F. Incidentally, the diagram in Fig. 45 serves to 
show the character of the sea-bottom along the line of 
section ; it shows, for example, that the island of Bermuda 
rises as an isolated peak from water of great depth.^ 

After what has been explained in Chapter IV. respect- 
ing the effect of heat in altering the bulk of bodies, it 
will be understood that a body of warm water, like that 
of the Gulf Stream, can easily float upon water which is 
colder and therefore denser. When a mass of water is 
unequally heated, by raising its temperature below, or by 
lowering it above, currents are at once established ; and, 
if light matter, such as sawdust, be suspended in the liquid, 
the direction of these currents becomes very evident. 
Thus in Fig. 46, where heat is applied at the bottom of 
a vessel, the liquid becomes specifically lighter and therefore 
rises, whilst the surrounding colder water being denser, runs 
down in streams to supply the place of that which has 
ascended to the surface. This is, in fact, the ordinary way 

^ The caution which has ah-eady been given respecting the exaggera- 
tion of the vertical height in the diagrams, must not be forgotten. 



X'l 



THE SEA AND ITS WORK. 



17'/ 



in which heat is propagated through a body of h'quid, and 
the process is called co?ivection,^ to distinguish it from con- 
ductton, or the method by which heat is propagated through 
solid bodies. In conduction, the heat is passed on from 
particle to particle, and thus travels through the mass, while, 
in convection, the heated particles themselves move. Again, 
if a piece of ice be dropped into a tumbler of slightly warm 
water, a system of currents will also be established, as in 
Fig. 47. From the bottom of the piece of ice a clear 





Fig. 46. — Currents in water by heat. 



Fig. 47. — Currents in \va;er by culd. 



Stream of heavy cold liquid flows down the middle of the 
glass, like a stream of clear oil, while the neighbouring 
water, which is comparatively warm, flows upwards in 
currents nearer to the sides of the vessel. 

Unequal cooling or heating of the great natural masses 
of water will be competent to produce a circulation similar 
to that just described. During the recent voyage of the 
Challenger the temperature of the sea at different depths 

1 Convection, from Lat. con, and veho, I carry, the heat being 
carried by currents through the fluid mass. 



178 PHYSIOGRAPHY. [chap. 

was very carefully examined by means of instruments 
specially constructed to avoid sources of error. These ob- 
servations show that, as a rule, the temperature diminishes 
as you descend, just as was shown to be the case in the 
North Atlantic. Reference to Fig. 45 shows tliat the 
bottom-water of that part of the ocean has a temperature 
only a little above 35° F. ; while, in other places, it is still 
lower, and may even descend below the freezing-point of 
fresh water.^ It appears that the presence of such cold 
water in the deeper parts of the ocean, even in tropical 
regions, can hardly be explained otherwise than by assum- 
ing a grand movement of water from the polar towards the 
equatorial regions. Dr. Carpenter has brought forward 
much evidence to prove the existence of such a general 
oceanic circulation, and he refers the movement mainly to 
differences of density due to differences of temperature. 
The cold polar waters sink by their density and form a 
deep layer, which creeps along the ocean-floor towards the 
equatorial regions ; while the warmer and relatively lighter 
water floats on the surface in a contrary direction, or from 
equatorial towards polar areas. ^ By such means, a complete 

^ The freezing-point of water is lowered by the addition of common 
salt, and ordinary sea- water does not freeze until reduced to 28° '4 F. 

^ The character of this circulation will of course be greatly modified 
by the shape and .depth of the sea-bottom over which the cold water 
creeps. Thus, the southern part of the Atlantic basin communicates 
freely with the Antarctic Sea, and the influx of cold water is there- 
fore unimpeded ; but the northern part of the basin is contracted, and 
the principal channel through which Arctic water can flow southwards 
is the shallow channel between Greenland and Iceland ; hence the 
underflow of glacial water from the North will be much less than that 
from the South. This is still more strikingly shown by the shape of 
the great Pacific basin, where the communication with the Northern 
Polar seas is confined to the narrow and shallow channel of Behring's 
f -rait, through which very little glacial water can flow to the soutii. 



XI.] THE SEA AND ITS WORK. 179 

circulation might be established; and it has consequendy 
been said that every drop of water in the open ocean 
may, in course of time, be brought up from the greatest 
depths to the surface. Other meteorological conditions, 
however, may exert an influence of the same kind, as great 
as, or even greater than, that produced by difference ot 
temperature. Sir Wyville Thomson regards the influx ot 
cold water into the Pacific and Atlantic Oceans from the 
south as an indraught due to " the excess of evaporation 
over precipitation in the northern portion of the land 
hemisphere, and the excess of precipitation over evaporation 
in the middle and southern part of the water hemisphere." ^ 

It seems probable that ocean-currents are of no great 
importance as agents of denudation or of transport. A 
slow circulation of the entire mass of the ocean, brought 
about by such comparatively slight differences of density in 
the water of different parts of the ocean, as are here under 
consideration, might perhaps facilitate the dispersion of the 
finest sedimentary matter. Again, where the surface-currents 
strike upon the shore they must do something in the work 
of denudation, though as a rule this will be exti'emely slight: 
the eff'ect of currents, indeed, is not so much to abrade the 
land as to carry off the results of its abrasion by other means, 
and to distribute the finely-suspended matter, far and wide, 
over the floor of the ocean. 

In addition to the movements of the sea which have 
been already noted in this chapter — the wind-waves, the 
surface- currents, and the general circulation — it must not 

Hence it is believed by Sir Wyville Thomson that the greater part oi 
the cold bottom-water in the North Pacific, and a good deal of tnai lU 
the North Atlantic, is an indraught from Antarctic, and not from Arctic 
seas. 

^ Proceedings of the Royal Society, vol. xxiv. No. 170, p. 470. 

N 2 



i8o PHYSIOGRAPHY. 

be forgotten that the ocean is subject to that grand 
rhythmical movement which was referred to in the first 
chapter. We saw, when standing on London Bridge, that 
the water regularly ebbed and flowed; and, what it does 
there, it does at every point along our coast. Twice in 
every four-and-twenty hours the margin of the sea rises, 
and twice it falls, so that its level is constantly shifting up 
and down. And yet it is a common practice to say that a 
given elevation is so many feet above the sea-level. Such 
a statement assumes that the standard taken is neither high- 
water mark nor low-water mark, but the mean level betAveen 
the two ; the water rising, at one time, as much above our 
standard level as it falls, at another time, below it. The 
Ordnance Survey has fixed its datum Hue, or standard from 
which all heights are measured, as tlie mean tide-level at 
Liverpool, The level of high water at London Bridge, 
which is sometimes taken as a standard, is called, from 
the Trinity House, " Trinity High-water mark." 

As the cause of the tides is to be found outside our 
earth, its explanation must be deferred to a later portion of 
this work. It is sufficient to remark, in this place, that the 
great tidal- wave, which travels round the earth, is an oscilla- 
tory wave, and not a wave of translation ; the water simply 
rising and falling, but not moving onwards. While, how- 
ever, this is true of the tidal wave in the ocean, it must be 
borne in mind that, in narrow seas, it becomes converted 
into an actual wave of translation. Where the channel is 
contracted, as in a narrow strait, the tide may produce a 
rapid rush of water, or a race. If, again, the tidal wave 
rolls into a narrow estuary, the water becomes heaped up, 
isA produces a sudden rush into the channel of the river : 
such a wave is called a bore, and is well seen in the Bristol 
Channel, at the mouth of the Severn, where at certain 



XI.] THE SEA AND ITS WORK. . i8i 

seasons the head of water attains to as great a height as 
forty feet. 

In the estuary ot a tidal river, the tide periodically agitates 
the water ; and thus hinders deposition of sediment. The 
flow of the river seawards is, however, checked every time 
the tide comes in, and sediment is then deposited ; hence, 
bars, or banks of sand, are common at the mouths of rivers ; 
and, even in the estuary of the Thames, the shifting shoals 
indicate similar depositions. But, it has been shown in a 
former chapter, that the ebb-tide, by scouring out the 
estuary, prevents the formation of a true delta. The posi- 
tion of the principal sands in the estuary of the Thames 
between the Nore and Margate, is shown in Fig. 48, whi>."h 
is reduced from a part of the Admiralty chart. 

The sediment which the tidal water carries away from 
the mouth of a river at one part of the coast may be 
deposited at another point, and thus the sea may become 
a constructive agent charged with the formation of new 
land. Usually, however, the suspended matter swept away 
by the ebb-tide is carried out to sea, where it may be caught 
up by currents and thus drifted to a great distance. Hence 
the tides and currents assist greatly in distributing the solid 
matter derived from the waste of the land. 

Putting together what has been said in this chapter with 
reference to the action of the sea upon the land, it may be 
concluded that its work, on the whole, is a work of destruc- 
tion, yet not exactly like that of rain and rivers. To 
observe this difference, it must be borne in mind that marine 
denudation is not equally active at all depths of the sea. 
The waves, as explained above, indicate only superficial 
agitation, and have no effect on deep water. Most of the 
destruction wrought by the sea is consequently confined 



PHYSIOGRAPHY. 



[chap. 



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XL] THE SEA AND ITS WORK. , 183 

within narrow limits, not extending deeper than a few 
hundred feet, and being Tor the most part restricted to the 
zone of coast between high and low water-marks. At great 
depths, the abrasion by slow under-currents must be ex- 
tremely small, for dredgings have shown that, in deep seas, 
there are no large fragments of rock to assist in the work 
of demolition ; and, even if there were, the force of the 
current would probably be insufficient to move them. The 
great business of the sea is therefore confined to eating 
away the margin of the coast, and planing it down to 
a depth of perhaps a hundred fathoms. If this action 
went on for a sufficient time, the entire coast would 
be nibbled away, and Britain reduced to a great plain 




Fig. 49. — Plain of marine denudation. 

below the sea-level. The comparatively smooth surface 
which would be formed in this manner has been called by 
Prof. Ramsay a plain of inaruie denudation. Were such a 
submarine plain to be upheaved above the surface of the 
water, it would immediately be attacked by rain, frost and 
other atmospheric agents, and would eventually be chiselled, 
by these means, into a variety of physical features. It is 
believed that old plains of marine denudation may yet be 
detected in certain districts. Thus, if Fig. 49 represents a 
section across a country where the highest points may be 
connected by a plane, the edge of which is A B ; this plane 
surface, with a gentle slope seaward, probably coincides with 
the original plain of marine denudation, or is at least parallel 



284 PHYSIOGRAPHY. [CH. xt 

to it, the present irregular surface of the country being 
due to subaerial denudation. Denudation by the sea differs 
then from that effected by other agents, in that it tends to 
produce an approximately level surface, while subaerial 
denudation gives rise to superficial irregularities. 



CHAPTER XII. 

EARTHQUAKES AND VOLCANOES. 

All the natural agents described in the last three chapters, 
however much they may differ among themselves, agree in 
this — that they are, upon the whole, slow and certain agents 
of destruction. Rain and river, frost and thaw, wind and 
wave — all work in the same direction, persistently attacking 
the solid land and sweeping away its superficial substance. 
Not that a particle of this substance is annihilated. Every 
grain stolen from the land is sooner or later carefully depo- 
sited somewhere in the sea. But, still, this gradual transfer- 
ence of matter, from land to water, must ultimately result 
m the lowering of the general level of the land to that of 
the sea by the action of the rain and rivers ; and, in the 
subsequent paring down of the plain, thus formed, to the 
depth at which marine denudation becomes insensible. If, 
therefore, no hindrance were offered to the action of these 
agents, not only would a time come when every foot of the 
British Isles would be buried beneath the sea ; but, inasmuch 
as the volume of the sea is very much greater than that of 
the land which rises above the sea-level, if sufficient time 
were granted, all the dry land in the world would ultimate'y 
disappear beneath one universal sheet of water. 



i86 PHYSIOGRAPHY. [cuap. 

It is not difficult, however, to detect in the operations of 
nature, counterbalancing forces which are capable of upheav- 
ing the deposits that have been formed on the sea-bottom, 
and of piling up fresh stores of solid matter upon the surface 
of the earth. Among these elevatory, and therefore repara- 
tive, agents, the most important place must be assigned 
to those which give rise to earthquakes and volcanoes. 
After the occurrence of an earthquake, it is by no means 
uncommon to find that the level of the land has been shifted. 
Sometimes, it is true, the surface is depressed, but more 
commonly the movement is in the direction of elevation. 

Perhaps the best recorded example of such upheaval is 
that which was observed by Admiral Fitzroy and Mr. 
Darwin when examining the western coast of South America. 
This region is peculiarly subject to subterranean distur- 
bances, and in 1835 a violent earthquake, which destroyed 
several towns, was felt along the coast of Chile, extending 
from Copiapo to Chiloe. It was found, after the shock, 
that the land in the Bay of Concepcion had been elevated 
to the extent of four or five feet. At an island called Santa 
Maria, about twenty-five miles south-west of Concepcion, the 
upheaval was easily measured, vertically, on the steep cliffs ; 
and the measurements showed that the south-western part 
of the island was raised eight feet, while the northern end 
was lifted more than ten feet high. Beds of dead mussels 
were, in fact, hoisted ten feet above high-water mark; and 
an extensive rocky flat, previously covered by the sea, was 
exposed as dry land. In like manner, the bottom of the 
surrounding sea must have been elevated, for soundings all 
round the island became shallower by about nine feet. It 
is true, there was a partial subsidence shortly afterwards, but 
this was far from sufficient to neutralize the upheaval, and 
the net result showed a permanent elevation. It is considered 



XII.] EARTHQUAKES AND VOLCANOES. 1S7 

probable, that the greater part of the South American coast 
has been raised several hundred feet by a succession of such 
small upheavals. 

When an area is thus raised, the addition suddenly made 
to the mass of dry land may be very considerable, and will 
compensate for the effects of denudation continued through a 
long period. It was calculated, for example, by Sir C. Lyell, 
that,during an earthquake which occurred in Chile in 1822, a 
mass of rock more than equal in weight to a hundred thousand 
of the great pyramids of Egypt was added to the South- 
American continent. If a single convulsion of this kind can 
thus raise such an amount of solid land from beneath thewaters, 
it is obvious that these movements must be of great service 
in renovating the surface of the earth, and in bringing new 
material within reach of the ever-active agents of denudation. 
It is proper to remark, that an earthquake-wave is a vibration 
of the solid crust of the earth, which may, and constantly does, 
occur, without giving rise to any permanent change in its 
form. Nevertheless, the wave is often accompanied by 
movements of elevation, or of depression, which produce 
permanent alterations of level of considerable magnitude. 

An earthquake is just such a disturbance of the ground as 
would result from a sudden shock, or blow, given upwards in 
the interior of the earth, from which, as from a centre, v/aves 
or tremors may be propagated in all directions through the 
solid ground. In many cases, the shock is preceded or 
accompanied by a rumbling noise, like that of distant 
thunder, or by other sounds produced by the subterranean 
disturbance. The earthquake-wave, as it travels along, 
causes the ground to rise and fall, and frequently produces 
irregular fissures, which may close again and thus bury 
whatever has been engulfed, or may remain open as yawning 
chasms, and thus modify the drainage of the country. The 



1 88 PHYSIOGRAPHY. [chap. 

impulse may be transmitted through the earth to an enor- 
mous distance ; the great earthquake which destroyed 
Lisbon in 1755, having made itself felt, directly or indirectly, 
on the waters of Loch Lomond in Scotland. If the centre 
of disturbance is near the sea, the water is affected even 
more than the land, and the water-waves may be far more 
destructive than the earth-waves. News has recently reached 
this country of the terrible devastation wrought by the great 
tidal wave which followed the earthquake at Lima, Arica, 
Iquique and other points of the coast of South America 
in May, 1877. 

A good deal ot attention has been paid by Mr. R. Mallet 
to the study of earthquake phenomena, or Seismology,^ and 
he is led to conclude that the origin of the disturbance is 
usually not deep-seated in the interior of the earth, probably 
never exceeding a depth of thirty miles ; while in many 
cases, it is certainly much less. Thus he ascertained that 
the great Neapolitan shock of 1857 had its origin at a 
depth of only eight or nine miles beneath the surface. Dr. 
Oldham has since found that a great earthquake at Cachar, 
in India, in 1869, had its focus, or centre of impulse, at a 
depth of about thirty miles. 

Although earthquake-shocks are happily of rare occur- 
rence in this country, it must be remembered that, in many 
parts of the world, they are by no means rare phenomena; 
and, probably, it is not overstating the case to say that earth- 
quake shocks occur, on an average, about three times a 
week. During the year 1876, for example, no fewer than 
104 earthquakes are recorded in Professor Fuchs's Annual 
Report; and, in the preceding year, as many as 100 days 
were marked by the occurrence of shocks. But, in addition 
to these, there are no doubt many slight disturbances, in 
^ Seismology, from o-eKT/K^y, seismos, a shock or earthquake. 



XII.] EARTHQUAKES AND VOLCANOES. 189 

unfrequented districts, which are never recorded in such 
reports. The total effect produced by the causes of such 
disturbances must consequently be far from insignificant, 
even in the course of a single year. 

Subterranean disturbances which commence merely v/ith 
quakings of the ground often terminate with the forcible 
ejection of heated matter from the interior of the earth 
A rent may be produced at some weak point, and this crack 
then serves for the passage of large volumes of steam and 
other vapours, with showers of red-hot ashes, accompanied 
or followed by streams of molten rock. The solid materials 
are shot forth into the air, and fall in showers around the 
mouth of the orifice; where they form, by their accumulation, 
a cone-shaped mound or hill. Such a hill is called a volcano^ 
or popularly a " burning mountain." It must be borne in 
mind, however, that it does not " burn," in the sense in 
which a fire burns, but it merely offers a channel through 
which heated matter is erupted from below. It differs again 
from an ordinary mountain, in that it is simply a heap of 
loose materials and melted matter, which has been piled up, 
layer after layer, around a hole leading down to the interior 
of the earth. Hence, if a volcano were cut through, it would 
probably present a section something like that shown in 
Fig. 50. Here a channel, a, has been opened through 
strata, h, b, originally horizontal, and the ejected matter has 
fallen all round the orifice in conical layers, each forming a 
mantle thrown irregularly over the preceding layer, and 
sloping in aU directions away from the central chimney. 

At the mouth of the volcanic pine, there is usually a 
funnel-shaped opening known as the crater. Fragmentary 
materials falling back into this cup, or rolling in from the 
sides, form layers which slope toivards the vent and there 

^ Volcano, Italian vulcano, from Vulcan, the Roman god of fire. 



I go 



PHYSIOGRAriiY. 



[chap 



fore in the opposite direction to the dip of the volcanic beds 
which make up the mass of the mound. A section of a 
cone of loose cindery materials is given in Fig. 51, and 
shows the difference of dip just referred to. The molten 
matter which wells up the throat of a volcano, cements the 
loose ashes and cinders into a compact mass, where it comes 
in contact with them, and thus forms a hard stony tube 
lining the volcanic chimney.- 









y 




Diagrammatic Section of a Volcano 



At the beginning of an eruption, clouds of steam are 
copiously belched forth, showing that water has its part to 
play even in these fiery phenomena. The steam generally 
issues spasmodically, each puff giving rise to clouds which 
shoot up to a great height, and are either dissipated or con- 
densed in torrents of rain. Associated with the steam are 
various gaseous exhalations, most of which, however, are not 
combustible. Hence, the appearance of a column of flame, 
often said to be seen issuing from a volcano, must generally 



XII.] EARTHQUAKES AND VOLCANOES. 191 

be an illusion, due to illumination of the vapours, partly by 
the sparks and red-hot stones and ashes shot out at the 
same time, and partly, by reflection from the glowing walls 
of the pipe and from the surface of the molten mattei 
below. In the early stages of an eruption, huge fragments 
of rock may be ejected ; for when, after a period of repose, 
the pent-up steam and gases at last gain vent, they violently 
eject the materials which have accumulated in the throat of 
the chimney, and choked its opening. Masses of rock, some 
as much as nine feet in diameter, are said to have been cast 
forth from the great volcano Cotopaxi, in Quito, during the 




Fig. 51. — Diagiamnuitic Section of a Cinder Cone. 

eruption of 1553, and to have been hurled to a distance of 
more than fifteen miles from the mountain. 

During an eruption, ashes are commonly ejected in great 
quantity, but it must be borne in mind that the m.aterials 
so-called are very different from the partially-burnt fuel of 
the domestic hearth. Volcanic ashes are, in fact, nothing 
but fragments of lava, or partially-fused rocky matter. When 
jets of this lava are shot forth from the volcano, the hquid is 
broken up by the air, and so splashed about that it falls in 
drops, which harden into small spongy fragments, resembling 
ashes and cinders. In some cases, the lava is broken into 
such fine particles that it is known as volcanic dust or sand ; 



192 PHYSIOGRAPHY. [chap. 

dense showers of such dust have been known to darken the 
sky for miles around the volcano, and have been wafted 
by winds for even hundreds of miles. It is an interesting 
fact, shown by the examination of the sea-bottom by the 
Challenger^ that volcanic detritus is almost universally 
dist':lbuted over the floor of the deep sea. 

When the steam, which is abundant in most eruptions, 
condenses in torrents of rain, the volcanic dust is frequently 
worked up into a hot mud which rolls down the hill in a 
sluggish stream, burying everything before it. Herculaneum 
was sealed up by a crust of volcanic mud discharged from 
Vesuvius ; while Pompeii was overwhelmed by a vast 
accumulation of dust and ashes during the same eruption. 

The partially-molten rock called lava rises up in the 
volcanic pipe, and may eventually run over the lip of the 
crater, or force its way through cracks in the hill, forming 
red-hot streams which generally present a consistence some- 
thing like that of treacle. These lava-torrents are often of 
great magnitude ; thus, it was estimated that in the famous 
eruption of Skaptar Jokul, in Iceland, in 1783, the mass or 
lava brought up from subterranean regions was equal to the 
bulk of Mont Blanc, The lava rapidly cools on the surface, 
though long retaining its heat beneath the protecting crust ; 
and, ultimately, the entire mass solidifies, forming a hard 
rock, more or less like a slag from an iron-furnace. In 
different specimens, however, the lava exhibits great varia- 
tions ; some being dark-coloured and comparatively heavy, 
while others are lighter in colour and much less dense ; in 
some cases the rock is compact, while in others it is spongy 
or cindery, when it is said to be scoriaceoiis. The little 
cavities, or vesicles, in this scoria} or cellular lava, are 
formed by the disengagement of bubbles of gas or vapour, 
' Scoria, volcanic cinder, from Lat scoria^ "dross." 



XII.] EARTHQUAKES AND VOLCANOES. 193 

when the matter is in a pasty condition ; just as the porous 
texture of a piece of bread is due to the presence of 
bubbles of gas evolved by the fermentation of the yeast. 
The stone largely used for scouring paint under the name of 
pumice^ is a lava of very porous texture ; its name recalling 
its origin as the froth or scum of lava. Sometimes, the 
masses of lava, which are tossed into the air, are rotated 
during their flight, and fall as more or less rounded bodies, 
known as volcanic bombs. Occasionally a very liquid lava may 
be caught by the wind, and drawn out into delicate fibres, like 
spun glass ; this beautiful form is very abundant at Kilauea, 
a volcano in Hawaii, one of the Sandwich islands, where it 
is known as Pele^s hair, its name being borrowed from that 
of an old goddess who was supposed to reside in the crater. 
Other lavas again are vitreous, and strongly resemble dark- 
coloured bottle-glass, when they pass under the name of 
obsidian. This kind of lava was largely used by the ancient 
Mexicans for making rude knives and other cutting instru- 
ments ; and a hill in northern Mexico, formerly worked for 
this material, is still known as the Ce?-ro de Navajas (Spanisli 
" Hill of Knives.") 

It often happens that the lava which wells up in the pipe 
of a volcano, breaks by its sheer weight through the rim of 
the crater, or even breaches one side of the conical hill. 
Thus Fig. 52 represents a group of small extinct volcanoes 
in Central France, showing cones which have been broken 
through in this way. In some cases the flanks of the cones 
are rent, and lava is then injected into the cracks, forming, 
when cold, huge rocky ribs known as dykes. In other cases, 
the chimney gets choked by a plug of hard lava, and new 
vents may then be opened on the side of the cone. Fig. 53 

1 Pumice, from the Lat. pumex, formerly spumex, allied to sputna^ 
"froth."' 

O 



194 



PHYSIOGRAPHY. 



[chap. 



is an ideal section of a volcano, showing the dykes of lava 
running through the stratified deposits, and also showing 
two minor cones a I', thrown up at points where the volcanic 
matter has been able to force its way to the surface. Mount 




Fk s'' — r reached Volcanic Cunes Ausergne 



Etna is remarkable for having its flanks studded with para- 
sitic cones, some of which are of considerable size, one 
being upwards of 900 feet in height. 

After a volca.no has long been silent and the large crater 




Fig. 53. — DiagramTiaiic S-^ctwii of Volcano, wuh Dykes and Minor Cones. 



has been more or less filled, partly by ejected materials 
which had fallen back into the cavity, during the last 
eruption, and partly by matter washed in by rain, renewal 
of activity through the old channel may give rise to the 



XII.] 



EARTHQUAKES AND VOLCANOES. 



195 



formation of a new cone seated within the old crateral 
hollow. Great changes may indeed be effected in the 
character of a volcano by successive eruptions, new cones 
being thrown up at one time, and old ones obliterated at 
another. Fig. 54, shows the summit of Vesuvius as it 
appeared in 1756, when' there were no fewer than three 
separate cones, one within another, encircling as many 
craters. But about ten years afterwards the summit pre- 
sented the form shown in Fig. 55, where a single cone rises 




Fig. 54 — bummit of Vesuvius in 1736. 



from the floor of the great crater. The curious stages 
through which a volcano may pass are well illustrated by the 
story of Vesuvius. 

Rather less than two thousand years ago, that mountain 
was as peaceful as Primrose Hill is at the present day. It 
seems from all accounts to have had a very regular conical 
shape, with a crater about a mile and a half broad. Yet its 
shape led hardly any one to suspect that the mountain was a 
slumbering volcano. Wild vines were growing over the 

o 2 



196 



PHYSIOGRAPHY. 



[chap. 



sides of the crater, and it was in the natural fortress formed 
by this great amphitheatre that Spartacus the Thracian, with 
his httle band of gladiators, took up his position at the 
beginning of the Servile War in the year 72 e.g. Earth- 
quakes, as already pointed out, are often the heralds of 
volcanic eruptions ; and the first notice which the old 
dwellers around Vesuvius received of its renewed activity 
was from a series of earthquakes which began, as far as we 
know, in a.d. 63, and continued intermittently for about 




Fig. 55. — Summit of Vesuvius in 1767. 



sixteen years. These disturbances culminated in the great 
eruption of a.d. 79, which has been described in two letters 
written by Pliny the Younger to Tacitus. The elder Pliny, 
the author of the famous Hisio7-ia Natiwalis, was, at that 
time, in command of the Roman fleet off Misenum. On the 
24th of August a cloud of unusual size and shape was seen 
hanging over the mountain. It is described as having had 
the form of a huge pine-tree ; and similarly- shaped masses 
of cloud usually accompany the eruptions of Vesuvius. An 



xn.J EARTHQUAKES AND VOLCANOES. 197 

enormous column of steam, mingled with ashes and stones, 
shoots up from the crater to a height of a thousand or 
twelve hundred feet, where the clouds spread out in hori- 
zontal masses, some miles m breadth, while the ashes and 
stones fall down in showers. Attracted by so curious a 
sight, the elder Pliny went to Stabiai, about ten miles from 
Vesuvius, but his eagerness to witness the spectacle cost 
him his life. His nephew, who stayed at Misenum, de- 
scribes the scene — the showers of ashes, the ejection of 
redhot stones, the movement of the land, the retreat of the 
sea, and other phenomena characteristic of the eruption of 
a volcano attended by an earthquake. So vast were the 
quantities of ashes and other fragmentary matter ejected, 
either dry or mixed with water, that the unfortunate cities of 
Herculanieum, Pompeii, and Stabiae were buried beneath 
deposits, in some places, thirty feet in thickness. It is 
doubtful, however, v/hether any true lava was erupted on 
this occasion. From that date to the present day, Vesu- 
vius has been more or less active, though sometimes quiet 
for considerable intervals. During the great eruption just 
referred to, the south-western side of the original cone was 
destroyed, but the half which was then left has remained in 
existence up to the present time, and forms the semi-cir- 
cular hill known as Monte Somma. Fig. 56 is a view 
of Vesuvius half encircled by the cliffs of this ancient 
crater.^ 

When a volcano is situated near the coast — and by far 
the larger number of existing volcanoes are so situated — the 
ashes may be aiiowered into the sea, or be borne thither by 
the wind, and may, in this way, get mixed with the detrital 
matter which is spread over the sea-bottom. A curious series 

^ .Figs, 51 to 57 are taken, by Prof. Judd's permission, from the Lile 
Mr. Poulett Scrope's work on Volcanoes. 



igS 



PHYSIOGRAPHY. 



[chap. 



of deposits may thus be produced, consisting partly of 
materials worn away from the land by the action of the 
•uater, and partly of matter ejected from subterranean 




Fig. 56. — Vesuvius and Monte Somraa. 

sources. In some cases, volcanic outbreaks take place 
actually beneath the sea, and the matter thrown up becomes 
mixed with the remains of shell-fish and other marine 
ori^anisms. Submarine volcanoes occasionally give rise to 




Fig. 57- — Graham Island, 1S31. 



new land, the erupted matter being piled up in sufiicieni 
quantity to form an island rising above the waters. Thus, 
in the year 1831 an island, which Admiral Smyth named 



XII.] EARTHQUAKES AND VOLCANOES. 199 

Graham Island (Fig. 57) appeared in the Mediterranean, 
between Sicily and the coast of Africa, where there had 
previously been more than 100 fathoms of water. The 
pile of volcanic matter forming tliis isle must have been 
upwards of 800 feet high, for the highest part of the island 
was 200 feet above water ; while the circumference of the 
mass of land was nearly three miles. After it had stood 
above the waves for about three months, the island entirely 
disappeared. 

It is probably that a great deal of the force by which 
volcanic products are brought to the surface is due to the 
conversion into steam of water which, in some way or other, 
obtains access to the deep-seated molten rocks; but, whether 
this is the sole source of volcanic energy or not, is uncertain. 
Numerous hypotheses have been advanced to explain the 
source and origin of the molten matter itself. Some of 
these attempts at explanation refer the heat to chemical 
and some to mechanical causes ; while others assume that 
it is merely the residue of the heat which the earth origi- 
nally possessed, if, as seems likely, it existed at one time in 
a state of fusion. Dismissing, however, these vexed ques- 
tions, it is sufficient to remark that some source of heat 
unquestionably does exist in the earth beneath our feet. 

If a thermometer be buried in the ground at a depth of 
only a few inches below the surface, it is found to be 
affected by all superficial changes of temperature, and its 
indications show that it is cool at night and warm in the 
day, cold in the winter and hot in the summer. But 
plunged deep into the ground, or placed in a deep, cellar 
or cavern, these variations disappear, and one uniform 
temperature is registered under all circumstances. What 
that temperature is will depend principally on the climate of 



2GO PHYSIOGRAPHY. [cH. xii. 

the locality, the constant temperature being nearly the mean 
temperature of the surface. 

On going still deeper, the heat is found to increase; and, 
at the bottom of a deep mine, it is generally so warm that 
the miners are glad to discard most of their clothing. At 
present, the deepest mine in this country is the Rosebridge 
Colliery, at Ince, near Wigan, which has reached a deptli of 
2,445 f^^t. Experiments on the temperature at different 
depths, while sinking this pit, showed that the average 
increase is about i° Fahr. for every fifty-four feet. In other 
sinkings, somewhat different results have been obtained, the 
rate of augmentation being affected by the character of 
the rocks bored through and by the position which the 
strata occupy; whether, for example, they are inclined or 
horizontal. Thus at the Astley pit at Dunkenfield in 
Cheshire the rate was found to be i° for every seventy-seven 
feet, but this appears to be unusually slow. Perhaps it will 
not be far wrong to assume that the average increase is i° 
for every sixty feet : such at least is the rate which was 
adopted a few years ago by the Royal Coal Commission in 
their calculations. 

Even the deep sinking at the Rosebridge Colliery is but 
the veriest dent in the earth's surface compared with the 
actual radius of the globe. It gives therefore but scant 
information respecting the temperature of the deep-seated 
portions of the interior ; but, assuming such a rate of 
increase to continue, it is evident that at the depth of only 
a few miles the heat would be sufficient to fuse any known 
rock. It is true that the melting-point of a solid body may 
be greatly modified by pressure ; and it is obvious that, at 
great depths, the pressure must be prodigious. Nevertheless, 
the eruption of lava from volcanic vents sufficiently shows 




1 1 ' 

Ml 



1\>>i 





Fig. 58. — Beehive Geyser, Yellowstone Park, Colorado. (Hayden. 



302 PHYSIOGRAPHY. [chap. 

that, whatever the general state of the earth's interior, there 
must be at least local masses of molten rock. 

Additional evidence of the existence of heat at great 
depths is furnished by the temperature of the water yielded 
by certain springs. Some of the hot springs at Bath, for 
example, have a temperature of 120° F. Still hotter springs 
occur in many countries ; and, in volcanic districts, even the 
boiling-point is occasionally reached. The most remarkable 
of these hot springs are those known in Iceland as Geysers. 
Jets of boiling water with clouds of steam are intermittently 
thrown high into the air with great force and accompanied 
with loud explosions. The water generally holds silica in 
solution, as mentioned on p. 124, and this siliceous matter 
is deposited around the mouth of the hole as an incrustation 
called sinter. Although the Geysers of Iceland are best 
known, similar springs are found in New Zealand, and also 
in the Rocky Mountains of North America. Fig. 58, repre- 
sents a geyser in the -Yellowstone Park, described by Prof 
Hayden. No fewer than 10,000 hot springs, geysers, and 
hot lakes are said to exist within the area of the Yellowstone 
Park. The geyser, here represented in action, throws jets of 
hot water to a height of something like 200 feet. 

In some localities, hot water issuing from the ground is 
mixed with earthy matter ; and streams of thick mud accumu- 
late round the openings, so as to form conical hills, known 
as salses, or ftitid volcanoes. Such eruptions of mud, varying 
considerably in consistency and in temperature, occur, for 
example, in the Crimea and on the shores of the Caspian 
Sea. In other cases, hot vapours issue from cracks in the 
ground, as at the Solfatara, near Naples, where the vapours 
are charged with sulphur. A large industry has sprung up 
in the Tuscan Maremma, by utilizing the hot vapours which 
issue from smoking cracks, known as soj/ioni, and contain 



XII.] EARTHQUAKES AND VOLCANOES. 203 

particles of boracic acid which are used in the preparation 
of borax. 

Most of the phenomena just described, are probably to be 
regarded as representing the lingering remains of volcanic 
activity. When a volcano has become extinct, the effects 
of subterranean heat in the locality may still manifest them- 
selves in a subdued form, in such phenomena as those of hot 
springs. Many volcanoes, however, which appear at the 
present day to be perfectly quiet, are merely dormant, and 
may break forth with renewed activity at any moment. The 
early history of Vesuvius, as already pointed out, shows 
that a volcano, after being silent for ages, may suddenly 
start forth into fresh life. 

There are few better examples of an area in which volcanic 
action must have been rife on an enormous scale at a com- 
paratively recent time, than that furnished by the Auvergne 
and the neighbouring districts in Central France. There 
the traveller may see hundreds of volcanic cones, known 
locally as " puys," still preserving their characteristic shape, 
in spite of long exposure; there, too, are the streams of 
lava just as they flowed from the craters, or burst through 
the sides of the cones (Fig. 52), whilst thick sheets of old 
lava and beds of ash are spread far and wide over the sur- 
rounding country. The district known as the Eifel, on the 
west bank of the Rhine, between Bonn and Andernach, 
offers equally striking examples of extinct volcanoes. 

Even in the British Isles, it is not difficult to trace the 
remains of ancient volcanic outbursts, although these are not 
so fresh and well-marked as those just mentioned. Sheets 
of lava are found in the north-eastern part of Ireland, espe- 
cially in the county of Antrim where the remarkable scenery 
of the Giant's Causeway is due \o the fact that some of the 
old lava has split up into columns, not altogether unlike 



204 PHYSIOGRAPHY. [cH. xil. 

those into which a mass of starch sphts during drying- 
Similar evidence of volcanic action may be found in Scotland, 
whilst in North Wales there are extensive remains of eruptive 
rocks ; the state of fiery activity which they indicate dates 
back, however, to a very remote period of geological history. 
Yet it must not for a moment be supposed that any volcanic 
product still exists, as a crater, amongst the volcanic hills of 
Wales. So great mdeed has been the disturbance and de 
nudation of this part of the earth, that the old surface has 
long ago been swept away, and its present shape bears little 
or no relation to its form at the period of eruption. It is 
true, for example, that the summit of Snowdon is formed of 
volcanic rocks, yet the mountain, in its present form, does 
not in any way represent an old volcanic cone. 

Without pursuing this subject further, enough has been 
said to prove that peaceful as our islands now are, they have 
again and again been the scene of violent volcanic disturb- 
ances. Fire, indeed, has played as important a part as 
water in the geological history of the British Isles ; and it 
is highly probable that, at a depth, which, as compared with 
the diameter of the earth, may be justly termed insignificant, 
even the peaceful valley of the Thames is underlaid by an 
ocean of molten rock. 



CHAPTER XIII. 

SLOW MOVEMENTS OF THE LAND. 

Such movements of the land as those which accompanied 
the South American earthquakes, referred to in the last 
chapter (p. i86), must have been brought about by the com- 
paratively sudden action of subterranean forces. But the 
land is subject not only to a rapid rise and fall of this kind, 
but also to local elevations and depressions so gradual as to 
escape ordinary observation. Special means, indeed, are, in 
most cases, needed to detect these slow changes of level, and 
to measure their extent. Yet it is probable that such gradual 
oscillations of the land are, in the long run, of far greater 
importance in the economy of nature than those abrupt 
movements which occur spasmodically. It will be shown 
in a subsequent chapter, that every foot of solid ground 
within the area of the Thames basin has, at some time or 
other, been buried beneath the sea ; and it is therefore clear 
that elevatory forces must have been at work to lift up the 
sea-bed and expose it as dry land. Nor has such move- 
ment been effected once only. Any one who seeks to read 
the history of the rocks in the basin of the Thames will 
be driven to conclude that the level of the land has changed 



2o6 PHYSIOGRAPHY. [chap. 

again and again, rising at one time and falling at another. 
And probably, such changes have been effected, for the most 
part, quietly rather than violently ; by slowly-acting forces 
working through long periods of time rather than by sudden 
disturbances. 

It would, perhaps, be difficult to point to any clearer proof 
of such gentle oscillations of level having taken place within 
the memory of man, than that afforded by some well-known 
ruins on the shore of the Bay of Naples. Although this 
illustration has been used by Sir Charles Lyell and other 
writers, it is, nevertheless, well worth referring to again, since 
it shows most instructively the kind of evidence on which 
• geologists rely in proof of the instability of the surface of 
the land. 

About the middle of the last century, the attention of some 
Italian antiquaries was attracted by three stone columns, 
almost concealed by a growth of bushes behind a villa very 
near to the sea shore, in the western part of Pozzuoli, a town 
seated on the Bay of Baise, about seven miles from Naples. 
These columns were buried to a considerable height in an 
accumulation of soil, the removal of which brought to light 
the ruins of a magnificent building. A square floor, paved 
with marble, and measuring seventy feet in the side, showed 
the magnitude of the central court. This area had originally 
been covered with a roof supported by forty- six noble columns, 
some wrought in granite and some in marble, still remaining 
more or less perfect. It was assumed by the antiquaries of 
cue day, apparently on very slender grounds, that the build- 
ing had been a temple dedicated to Serapis, an Egyptian 
divinity, whose worship had been introduced into Rome. 
Just behind the building is a hot spring, from which water had 
been carried, through a marble channel, to a number of small 
apartments built around the central court. This has led to the 



XIII.] 



SLOW MOVEMENTS OF THE LAND. 



207 



suggestion that the building, instead of having been a temple 
of Jupiter Serapis, was nothing more than a magnificent 
bathing establishment. Be that as it may, however, it is 
convenient to refer to it for geological purposes under it« 
well-known name of the Temple of Serapis. To geologists 
its interest centres in the three tall columns by which the 



Y/fNT£R ir 



BOR/NOS 

BY 

S£A SHELLS 



It/CRUSTATWM 



\ OF WATER 



UPPER fLOOR 



Fig. 59. 



- i.VW£ff, PL 00 R. 

-Marble Column from Temple of Jupiter Serapis. 



building was discovered, and which are the only pillars, out 
of the original forty-six, still standing. Each column, though 
upwards of forty feet high, has been carved out of a single block 
oi green marble, Fig. 59. Up to the height of about twelve feet 
from the base, the colums are smooth \ but, above this level, 
each pillar is marked by a band of deep pits, the band being 



2o8 PHYSIOGRAPHY. [chap. 

about eight feet in breadth. Each pit is a pear-shaped hole, 
and at the bottom of the hole, which is the widest part, there 
may generally be found the two halves, or valves, of a shell 
not unlike that of the common mussel. Exactly the same 
kind, or species of shell-fish^ as that represented in the 
cavities of the marble, is found to-day living in the Medi- 
terranean, where it may be seen boring its way into lime- 
stone rocks, much in the same manner that the so-called 
" shipworm " bores into timber. There is no difficulty then, in 
understanding that the perforations in the columns of the 
temple are the work of boring shell-fish. But it is clear that 
the columns, when attacked, must have been washed by the 
sea, for the shell-fish could not have lived in the holes when 
left high and dry above water. The fact is thus established, 
that the part of the marble pillars bored by the shells must 
have been immersed in the sea, long enough for the creatures 
to have drilled the multitude of holes which are now visible. 
Here then is evidence of a considerable alteration in the 
relative level of land and water. It is obvious, however, 
that such an alteration of level may have been brought 
about in one or other of two ways : either the sea may have 
been raised, or the land may have been carried down. At 
first sight it appears much more likely that so mobile a 
thing as the sea has changed its level, than that the surface 
of the solid earth should have shifted its position. And yet 
it needs scarcely a moment's consideration to show that 
any local alteration of sea-level cannot possibly have 
occurred. For, supposing the surface of the sea to have 
been raised so as to reach the zone of shell-burrows, the 
water must then have been forced up into a great heap ; but 
directly this heap was formed, the particles at the top would 
press upon those below, and thus urge theui down the 

^ Lithodovnis Jactyhis. 



xiil.] SLOW MOVEMENTS OF THE LAND. 209 

sloping sides until they reached the general level. In fact, 
the freedom of motion which the liquid molecules enjoy 
renders it impossible to produce anything like a hump of 
water, except temporarily. As soon as the surface of the 
liquid is raised at one point, it tends to fall down again 
until a common level is restored. Hence if the sea rose, 
and kept its high level around the bases of the columns, the 
rise could not have been confined to the Bay of Naples, but 
must have been part of a general rise of the surface of the 
ocean, to the same extent, all over the world. The difficulty, 
however, of finding a source for so vast an amount of 
additional water, as this general rise would imply, is 
alone an insuperable objection to this hypothesis. But 
geologists have abundant other reasons for the conclusion 
that, in such cases, it is the land and not the sea that has 
shifted its level. 

It appears then that the marks left by the boring shell- 
fish on the columns of the temple of Serapis, upwards of 
twelve feet above sea-level, prove that the land on which 
these pillars stand must, at one time, have been depressed 
to that extent, and afterwards elevated to its present posi- 
tion. But the temple teaches much more than this. About 
five feet beneath the present marble floor of the build- 
ing, there are the remains of another floor ; and it 
seems only fair to suppose that the upper pavement 
was constructed after the lower one, which belonged to 
some earlier building, had been carried down to an in- 
convenient level by subsidence of the land. Such sub- 
sidence has indeed been going on in this locality within 
the present century ; for, when the ruins were first un- 
earthed, the upper floor stood much higher than it stands 
at present. Careful observations, in the early part of this 
century, shov/ed that the ground was sinking at the rate 

p 



2IO PHYSIOGRAPHY. [chap. 

of about one inch in four years, and some observers have 
given even a more rapid rate. The ruins stand close to 
the sea; and, as the pavement sank, the sea flowed in, 
so that, it is said, in 1S38, fish were daily caught within 
the temple, where, in 1S07, there was not a drop of water 
in ordinary weather. 

After what has been explained, it will be seen that the story 
of the temple may be interpreted somewhat as follows : — 
The present building was erected on the site of some older 
one, the floor of which had been carried down by sinking of 
the land. It may be fairly supposed that the pavement of 
the new building was at the sea-level, or thereabouts. In- 
scriptions found among the ruins prove that the temple was 
decorated by Septimius Severus and Alexander Severus, so 
that the building must have been in use during the third 
century of our era. But, by subsidence below the sea-level, 
water entered the court ; and deposits of solid matter from 
this water were gradually thrown down around the base of 
the pillars, mingled, at times, with layers of volcanic ashes. 
Traces of some of these deposits may still be seen adhering 
to the shafts, below the zone of borings (Fig. 59). As the 
lower parts of the pillars were buried beneath these accu- 
mulations, they were not attacked by the shell-fish which 
burrowed into the marble during the period of greatest 
depression. The depression was certainly gradual, but it is 
probable that the subsequent elevation may have been more 
rapid ; and it was, perhaps, partly effected during a violent 
subterranean disturbance in 1538, when a mountain, still 
called Monte Nuovo, was thrown up not far from the 
temple. It is certain, however, that none of the move- 
ments which have affected the temple could have been 
sufiiciently violent to overturn the columns that are still 
standing. 



XIII.] SLOW MOVEMENTS OF THE LAND. 211 

Such appears to have been the succession of events 
registered by these ruins. It is true, the Bay of Naples is 
in a region pecuharly subject to volcanic disturbances, but 
the slow movements of the land are by no means confined 
to such districts. Few countries in the world perhaps have 
been more free from such disturbances than Scandinavia. Yet 
direct measurement has shown that the northern part of th-is 
peninsula is slowly rising, while the southern part, curiously 
enough, appears to be suffering depression. In such a case 
as this, where movements in opposite directions are simul- 
taneously going on, it is useless to think of attributing them 
to any movement of the sea. For a change of sea-level 
implies, as already pointed out, a general change, whether 
of rise or fall;, and it is therefore absurd to assume a 
rise in one place and a fall in another, at the same 
time. 

In the British Isles, there is no lack of evidence to show 
that the level of the land has been frequently disturbed, 
though the oscillations within the memory of man are hardly 
so well marked as in the cases previously cited. Visitors 
to certain parts of tlie coast of Britain may note a terrace of 
sand and gravel, mixed perhaps with sea-shells, and having 
all the appearance of a deserted beach, which fringes the 
shore at a height far beyond reach of the highest tides. Such 
accumulations must have been formed along a line of shore, 
and afterwards elevated to their present position, whence 
they are termed 7-aised beaches. A raised beach is therefore 
an index of elevation of the land. And it appears that this 
elevation must have been effected, in part at least, since the 
country was inhabited. For the upraised deposits of silt 
which skirt the estuary of the Clyde have yielded relics of 
human workmanship, such as rude canoes, that Avere origin- 
ally buried in the mud and sand of the old estuary, 

p 2 



2M PHYSIOGRAPHY. [chap 

though they are now found several feet above high-water 
mark. 

Evidence of the depression of land in Britain is just as 
conclusive as that of elevation. It is sometimes possible to 
see, at low tide, in the estuary of the Thames, the remains 
of a vast forest, with the stools or stumps of the trees still 
rooted in the old soil, now submerged to a depth of perhaps 
twenty or thirty feet below high-water. The relics of this 
ancient forest show that it must have supported a rich growth 
of yew, pine, oak, alder, and other trees. Now, as these trees 
do not grow in water, it is evident that the land on which 
they flourished has been depressed. In some places, the 
remains of the old land-surface have been buried, to a depth 
of several feet, beneath alluvial deposits which have been 
thrown down by the river. When the marshy land on the 
coasts of Kent and Essex, bounding the estuary of the 
Thames, is cut through, the sections frequently expose the 
ancient peaty soil, rich in vegetable remains. This was the 
case, for example, during the progress of the Main Drainage 
Works, when deep trenches were cut through the marshes 
below Woolwich. It is not, however, only at the mouth of 
the Thames that such evidence of subsidence is to be 
found ; for similar submerged forests may be seen at low 
water at many points of the British coast, especially in 
Devonshire, Cornwall, and Wales. 

Raised beaches and submarine forests afford as good 
evidence of the rise and fall of the land as any to be got 
from the Temple of Serapis. But they are not the only 
evidence which the geologist can cite, to prove that the 
surface of the British Isles has suffered frequent changes ot 
level. Nor are they by any means the strongest. Indeed 
their value lies chiefly in the fact that the movements which 
they register are of comparatively recent date. That there 



XIII.] SLOW MOVEMENTS OF THE LAND. 213 

have been variations of level to a much greater extent, at 
more remote periods, is abundantly testified by the strata in 
almost any part of the country. London, for example, is 
seated on clay, which, as already pointed out, must have been 
deposited under water, in the state of mud. But as this 
clay contains, in many parts, the remains of marine shell- 
fish, such as the nautilus, there can be no doubt that the 
mud in such places must have been originally thrown down 
out at sea. The clays, sands, and other deposits below the 
London clay, already referred to under the name of the Lower 
London Tertiaries (p. 32), have been formed, some in salt and 
others in brackish water, as is testified by the character of the 
shells which they contain. As to the chalk, which lies in a 
mass of vast thickness immediately beneath these deposits, 
it will be shown, in a subsequent chapter, that it abounds 
with the remains of creatures which once lived in the depths 
of the sea. If, then, these rocks are, in great measure, 
nothing but old sea-bottoms, it is clear that great upward 
movement must have taken place to raise them into their 
present situation. 

But this is not all. The rocks have not only been up- 
lifted, but in many cases have been subjected to some dis- 
turbing action by which they have been more or less contorted. 
The section in Fig. 11, p. 31, shows that the strata beneath 
London lie in a gentle hollow, purposely exaggerated in the 
diagram, but still sufficiently marked in nature to suggest the 
name of " London Basin." Supposing they had been ori- 
ginally deposited in a depression of the sea-bottom, the 
layers would have been thrown down almost horizontally, as 
in Fig. 60; and not in curved layers of equal thickness as in 
Fig. 61, such as are really found in nature. The present 
position of these rocks is therefore explained by supposing 
that the strata were originally horizontal and have been 



2!4 PHYSIOGRAPHY. [chap 

thrown into a basin-like shape since their formation. The 
disturbance to which they have been subjected is yet more 
strikingly seen if a section be taken across, not only this 
basin, but across anotlier area of similar character, known 
as the Hampshire basin. Fig. 62 is a section from Abingdon 
in Berkshire, through Hampshire and across the Solent to 
the Isle of Wight ; the vertical scale being about twenty 
times greater than the horizontal scale. Here the strata, 
originally almost horizontal, have been thrown into a succes- 
sion of gentle undulations, rising to a crest in one locality 
and falling to a trough at another. There can be little doubt 
that the Eocene rocks, including the London clay, once 
spread over the whole surface of this chalk, but have since 
been removed from the high ground by denudation, leaving 



\ -^----— ^ 




Fig. 60. — Strata deposited in Fig. 6i. — Strata thrown into a 

a basin. basin-shape. 



isolated tracts separated by intervening patches of bare 
chalk. In the Isle of Wight, the beds have been so greatly 
disturbed that the chalk strata stand almost on end, as is well 
shown by the bands of black flints which run in almost 
vertical lines. Incidentally it may be remarked that when 
strata lie in this shape -^ they are said to form a synclinal^ 
and when in this form -~~- an anticlinal?' The strata in the 
south-eastern part of England have been but little dis- 
turbed \ but, among the old rocks of Wales and other parts 
of v/estern Britain, it is not uncommon to find the beds 

1 Synclinal, from aiv, sim, with ; and kXivco^ klino, to slope. 
* Anticlinal, from uvt\, anti, against ; and /i.ViVai. 



XIII.] 



SLOW MOVEMENTS OF THE LAND. 



215 



thrown into a succession of sharp anti- 
clinals and synclinals. 

During the disturbances to which strata 
have been subjected since their original 
deposition, it has frequently happened 
that the rocks have been broken and dis- 
located, as represented in Fig, 10, p. 30 ; 
where the series of beds, on one side of 
the fracture or fault, have been thrown 
down to a much lower level than that 
occupied by the strata on the opposite 
side. Even in an area so little disturbed 
as the London basin, such dislocations of 
the strata may be detected ; and, indeed, 
a considerable fault runs along part of the 
valley of the Thames below London, and 
throws down the beds on the north to the 
extent of 100 feet, or even more. Before 
leaving this subject, it may be well to 
mention that contortion and dislocation 
of strata may be due to squeezing at the 
sides, and not to the direct operation of 
forces acting immediately from below. 

From what has been said in this chapter, 
it will be seen thai deposits, formed origi- 
nally on the floor of the sea, have been 
hoisted above water, and now form the 
bulk of our dry land. The land is there- 
fore subject to a kind of circulation similar 
to that which has been already pointed 
out m the case of water. The water, it 
will be remembered, passed from the river 



2i6 PHYSIOGRAPHY. Lch. xiii. 

to the ocean, and back again from the ocean to the river in 
the form of rain. In hke manner, the soHd land is con- 
stantly being carried, piece by piece, into the sea. Here 
most of it is spread out upon the ocean floor, forming de- 
posits which will some day be raised as dry land ; and will 
then be once more attacked by the water, as soon as it rises 
above sea-level. The solid earth is, therefore, subject to a 
cycle of changes not less complete than that exhibited by 
the circulation of the waters. 



CHAPTER XIV. 

LIVING MATTER AND THE EFFECTS OF ITS ACTIVITY ON 
THE DISTRIBUTION OF TERRESTRIAL SOLIDS, FLUIDS, 
AND GASES— DEPOSITS FORMED BY THE REMAINS OF 
PLANTS. 

It has been seen that the fresh and salt waters which run 
upon, and beat against, the land, are constantly engaged in 
transporting the materials of which that land is composed 
from higher to lower levels. A comparatively insignificant 
fraction of these materials remains in the lakes, which lie in 
the course of some rivers; but, by far the greater part, 
sooner or later, reaches the sea. 

The solid deposits which thus accumulate on the sea- 
bottom are never exactly equal to the waste of the land, but 
are always of less, and frequently of much less, mass. For 
all the chief constituents of the land are more or less soluble 
in water; and hence, a larger or smaller proportion of the 
products of denudation reach the sea in a state of solution, 
and are diffused through the ocean, as the sugar in a drop of 
syrup would be diffused through a pail of water. Notably, 
dissolved carbonate of lime and silica are thus being con- 
stantly poured into the sea. 

Supposing that no influences were at work upon the earth's 



2i8 PHYSIOGRAPHY. [chap. 

crust, except those of rain and rivers, and of the sea ; then, 
as has been pointed out in Chap. XI., the ultimate result of 
their action would be the reduction of the solid land to a 
submarine plain. The waters covering this plain would be 
more or less completely saturated with the soluble materials 
extracted from the denuded rocks. Denudation, therefore, 
on the whole, not only diminishes the quantity of dry land, 
but also lessens the proportion of the solid to the fluid 
constituents of the globe. 

The tendency of the forces of upheaval is in the opposite 
direction. The fused rocks in the depths of the earth, 
which are vomited forth by volcanoes, are forced to the 
surface as liquids, and then take on the sohd form. There 
is a transference of matter from lower to higher levels, 
accompanied by an increase of the solid at the expense of 
the fluid constituents of the globe. Whether the proportion 
of dry land is, or is not, increased by volcanic action depends 
upon the locality of the vent, and the amount of matter 
thrown out from it. If the vent opens on dry land, the 
erupted matters will necessarily increase the mass of dry 
land ; while, if it break out beneath the sea, they may reach 
the surface, or not, according to their mass and the form 
which they assume. 

Supposing that no agents were at work upon the earth's 
crust except volcanoes (with concomitant movements of ele- 
vation and depression), the quantity of water in the ocean 
would remain substantially unaltered; but the proportion of 
the surface of the earth occupied by dry land to the area 
covered by water might be almost indefinitely increased or 
diminished. It is conceivable, for example, that the whole 
ocean, which, at present, occupies about three-fifths of the 
surface of the earth, might come to be contained in a few 
very deep lakes ; in consequence of the deepenmg of the 



XIV.] LIVING MATTER AND ITS EFFECTS. 219 

existing sea-valleys, and the elevation of the intermediate 
land areas. Or the contrary result might be brought 
about, by the depression of the existing land areas, and 
the raising of the sea-bottom by matter ejected from 
submarine volcanoes. 

Thus, so far as the mere transference of the matter of the 
crust of the globe is concerned, the tendency of volcanic 
action and of the elevatory forces is, on the whole, to com- 
pensate for denudation and depression ; and, it is con- 
ceivable, that the two processes should go on, for any 
assignable time, in such a manner that the proportion of 
land above the sea level to that below it should remain 
unchanged. But, in the operations of nature hitherto dealt 
with there is nothing to compensate for the gradual con- 
version of solids into liquids by denudation ; nor for such 
out-pouring of gases into the atmosphere as occasionally, 
if not always, accompanies volcanic action. 

Nevertheless, an agent, by which some of the gaseous 
and liquid constituents of the earth are, temporarily or 
permanently, reduced to the state of solids, is at work upon 
a prodigious scale. This agent is what is termed living 
matter ; or, less accurately, organic matter} 

The surface of the valley of the Thames is covered with 
prodigious multitudes, and seeming endless varieties, of 
the forms of this living matter, some of which we call 
plants, and others animals. But, notwithstanding their 
obvious differences, there are so many deep-seated points 
of agreement among the diversified forms of life that any 
plant or any animal will serve to illustrate the essential 

^ Less accurately, because all forms of living matter cann;t be 
strictly said to be organised. An organ is a part of a living body, the 
structure of which fits it for the performance of some special action, 
which is called \Xs function. The lowest forms of life possess no parts 
to which the term organ can be applied in this sense. 



220 PHYSIOGRAPHY. [chap 

characters of all plants and of all animals. Every one has 
seen a field of peas, thronged with pigeons. The peas will 
serve very well as examples of plants, and the pigeons as 
types of animals. 

The pea that may be extracted from a ripe peascod is a 
li'cing body, in which, however, the vital activities are, for the 
time, almost quiescent. Within the thin skin which envelops 
the pea, there is inclosed a perfect, though embryonic plant, 
composed of a minute stem with its root and leaves ; of 
which last, two, the cotyledoiis or seed-leaves, are so large and 
solid that they make up the chief mass of the infant pea- 
plant. 

Subjected to chemical analysis, the embryo plant yields 
certain complex bodies, composed chiefly of carbon, hydro- 
gen, oxygen, and nitrogen, which are known as protein com- 
pounds.^ Besides these it contains fatty matters ; woody 
substance (cellulose), sugar, and starch ; various salts of 
potash, lime, iron, and other mineral matters, including a 
considerable proportion of water. 

Examined with the naked eye, the soft substance of the 
young plant appears to be almost homogeneous ; the appli- 
cation of the microscope, however, shows that it is far from 
being really so ; but that, on the contrary, it has a very 
definite and regular structure. A delicate woody framework, 
or skeleton, is excavated by innumerable small cavities, each 
of which is filled by a semifluid matter, termed protoplasm,^ 
just as the honey fills the waxen cells of a honeycomb. 
Each mass of protoplasm, with its investing wooden wall, is 
technically termed a cell; and, inasmuch as part of the proto- 
plasm is distinguishable from the rest as a rounded nucleus, 

^ Protein, from irpairevw, protcuo, to have the first place. 
* Protoplasm, from irpwTos, protos, first ; and irXaffixa, plasma, forma- 
tive matter. 



XIV.] LIVING MATTER AND ITS EFFECTS. 221 

it is called a nucleated cell. The protoplasm contains tiie 
protein compounds, and the larger proportion of the saline 
and Avatery constituents of the plant. The cell-wall is mainly 
cellulose and water. Saccharine and fatty matters probably 
exist, diffused through the protoplasm, in all the cells; starch, 
in the form of granules, is to be found in most. 

The embryonic pea-plant, then, is no simple homogeneous 
mass, but is an aggregate, made up of a mukitude of distinct 
nucleated cells, each of which is essentially composed of a 
protoplasmic body invested by a cell-wall. The fact that 
this cell-aggregate is aUve, does not become manifest until 
the pea is exposed to certain conditions. But, every one 
knows, that if a pea is planted in the ground in moist and 
warm weather, it shortly bursts its coat. The seed-leaves 
enlarge and come to the surface ; while the rootlet grows 
into the soil. The stem shoots up; its minute and colourless 
leaves rapidly enlarge and become green ; new leaves are 
developed ; and, by degrees, a tall plant rises above the 
ground, the bulk and weight of which soon become many 
thousand times greater than those of the embryo. Then the 
plant blossoms, and in the centre of each flower is found a 
hollow organ, the pistil. From the walls of this, small 
bodies termed the ovules grow out, and each ovule contains 
a microscopic nucleated cell — the embryo cell. In the fer- 
tihsed ovules, the embryo cell divides and subdivides, each 
new cell growing until it becomes as large as, or larger than, 
that from which it proceeded; and, thus, by degrees, the 
single cell is converted into a cell-aggregate, which assumes 
the shape of the embryo plant. This, inclosed in the dis- 
tended envelope furnished by the ovule, is the pea — while 
the enlarged pistil becomes the peascod. 

Thus, the plant under consideration goes through a series 
of changes, the starting-point of which is the sim.ple nucleated 



222 PHYSIOGRAPHY. [chap 

cell (the embryo ccii) contained within the ovule, while its 
conclusion is the production of new embryo cells, every one 
of which may become competent to repeat the whole series. 
Each term of the series is a stage of what is called the de- 
velopment of the plant; and, if successive stages of this 
development are compared, it will be found that the plant 
becomes more complex the further its development ad- 
vances. The embryo plant, in the pea, is a more complex 
structure than the embryo cell in the ovule ; the blossoming 
plant is more complex than the young plant before flowering; 
and this increase of complexity is true, not only of the 
outwardly visible parts, but of the inward structure, of the 
growing plant. Nevertheless, it is to be observed, that 
the full-grown plant is as much an aggregate of nucleated 
cells, more or less modified, as is the embryo ; and every 
change in the form and size of the growing plant is, simply, 
the expression of the mode of growth and multiplication 
of the individual cells of which the body of the plant is 
made up. 

The process of evolution, from an extremely simple to a 
highly complex condition, thus exemplified by the pea-plant, 
is characteristic of living matter. For, although there is a 
superficial similarity between the growth of a plant and the 
tree-like form which some bodies assume in the act of crys- 
tallisation, as is well exemplified by the hoar-frost on a 
window-pane ; yet, a very slight examination shows that the 
two processes are, in reality, altogether different. When an 
individual crystal grows, the new matter is added upon its 
exterior ; and when crystalline bodies assume an arborescent 
form, the first crystal that is deposited does not grow into 
the crystal tree, but new crystals are added to the outside of 
the first, in such a manner, that the compound m?ss has 
a tree-like shape. But, when the embryo-cell grows, the 



XIV.] LIVING MATTER AND ITS EFFECTS. -223 

addition of matter takes place within its own substance, 
just as a piece of jelly swells by taking in water. And the 
primitively single cell becomes a cell- aggregate, not by 
attachment of strange cells, from widiout, to that which 
first existed ; but, by the growth and division of the primi- 
tive cell ; and the repetition of the processes of growth 
and division in the successive generations of new cells thus 
produced. 

There is another very striking difference between the 
growth of such not-living bodies as may be said to grow, 
and living growth. A crystal can grow, only if the materials 
of which it is composed exist, as such, in the liquid which 
surrounds it. A crystal of salt can grow only in a solution 
of salt; and a crystal of sulphate of soda, in a solution of 
sulphate of soda. ' 

It is quite otherwise with a plant. A single pea may not 
only develop into a large pea-plant, but may ultimately 
give rise to a multitude of peas as large as itself. In other 
words, the pea, in the course of its development, accu- 
mulates within itself many thousand times the quantity of 
protein compounds, of cellulose, of starch, of sugar, of fat, 
and of water and mineral salts, which it primitively contained. 
Nevertheless, of all these bodies, it is certain that none but 
the water and the mineral salts exist, as such, either in the 
air or in the soil. In fact, strange as it may seem, the soil 
is a superfluity. A pea will grow into a perfect plant and 
produce its crop of peas, if it is supplied with water contain- 
ing nitrate of ammonia and the phosphates, sulphates, and 
chlorides of potassium, calcium, iron, and the like, which it 
needs, and is freely exposed to the air and to sunshine. Under 
such conditions as these, it is obvious that the full-grown plant 
must be almost entirely composed of fluids and gases which 
have been transmuted into solid matters ; and that it has 



224 PHYSIOGRAPHY. [chap. 

manufactured the Miultifarious, and often highly complex 
chemical compounds, of which its body is composed, 
out of the comparatively simple raw materials supplied 
to it. 

In the case supposed, the fluid with which the pea is 
supplied contains only hydrogen, oxygen, nitrogen, phos- 
phorus, sulphur, and certain metallic bases; but another 
element, carbon, enters, largely, into every one of the manu 
factured articles which are to be found in the full-grown 
plant. The presence of this carbon, and its great relative 
amount, may be made manifest enough if the plant is strongly 
heated in a closed vessel, when the carbon remains, as a 
conspicuous mass of charcoal. Whence is this carbon de- 
rived ? Under the conditions defined, the only possible 
source of supply is the carbonic acid diffused through the 
atmo-iphere ; which, though it forms so small a percentage of 
the air, yet amounts to an enormous absolute quantity (p. 84). 
tn fact, it is known that, under the influence of sunlight, a 
green plant decomposes carbonic acid into its elements; 
and, setting free the oxygen, builds up the carbon, together 
with the nitrogen, hydrogen, and oxygen, and mineral matters 
derived from other sources, into the complex compounds of 
its own living substance. 

Thus the green plant transmutes the fluid and gaseous 
mattei-s, which it draws from the soil and from the atmos- 
phere, into the solid materials of its own body ; and thus, to 
a certain extent, reclaims the solids lost by aqueous solution 
and igneous decomposition. Under ordinary circumstances, 
the restoration of solid matter to the earth, thus effected by 
plant-life,, is only temporary. Even during life, the activity 
of the green plant, like all vital activity, is accompanied by 
the slow oxidation and destruction of its protoplasmic matter; 
and one of the products of this oxidation, carbonic acid, 



XIV.] LIVING MATTER AND ITS EFFECTS. 225 

is returned to the atmosphere. After death, the process of 
decay is accompanied by slow oxidation. The carbon 
goes ofif chiefly in the form of gaseous carbonic acid ; the 
nitrogen, in the shape of ammoniacal salts ; and the mineral 
salts are dissolved away by rain and carried to the general 
store of the waters. But if, by the overflow of a river, the 
plant should become silted up in mud, or carried away 
by floods and buried in the sea-bottom, the process of 
decay may be so slow and so imperfect, that its carbonized 
remains, often infiltrated with mineral matter, may be 
preserved as a ''/ossi'l,"'^ when the mud is hardened 
into a stone, and thus permanently contribute to the 
sohd land. 

So much for the plant ; let us now turn to the animal. The 
laid egg of a pigeon answers to the ripe pea. Within the 
shell, and suspended in the white of the egg, is the rounded 
yellow mass of the yolk, and on one side of the yolk is a 
small round patch — the cicatriciila? Though apparently 
homogeneous, the microscope shows that the cicatricula is 
made up of minute nucleated cells ; and this cell-aggregate 
is an embryo pigeon, just in the same sense as the little 
plant within the coat of the pea is an embryo pea-plant ; 
though it is far less like a pigeon than the latter is like a 
pea-plant. 

The embryo pigeon, like the embryo plant, contains pro- 
tein compounds, fats, mineral salts and water. The yolk 
in which it lies is composed of similar materials \ but neither 
starch nor cellulose enters into its composition. 

The cicatricula exhibits no more signs of life than the 

^ Fossil, Lat. fossilis, ixomfodio, to dig ; a term applied by old writers 
to anything dug out of the earth, and therefore including minerals, 
but now conventionally restricted to orgaaic remains. 

^ Cicatricula, Lat. diminutive of cicatrix, a scar. 

Q 



226 PHYSIOGRAPHY. [chap. 

young plant within the pea. It is in a quiescent state, and 
its activity must be roused by an external influence. This, in 
the case of the egg, is simply a certain amount of heat 
(which is ordinarily furnished by the warmth of the body of 
the parent), the supply of nourishment being yielded by the 
matter stored up within the egg itself, in the yolk and 
white. Under these circumstances, the cicatricula enlarges 
by the growth and multiplication of its cells, and rapidly 
extends over the surface of the yolk. Part of it rises up 
and becomes fashioned into a rude resemblance of the body 
of a vertebrated animal, in which the head, trunk, and tail 
gradually become more and more recognizable ; while the 
limbs grow out like buds, at first without much likeness to 
either legs or wings. 

As the yolk is used up in the construction of the embryo, 
it diminishes as the latter increases ; the young bird be- 
comes larger and larger, acquires its feathers, and puts on 
more and more completely the characters of a pigeon. At 
last, it leaves the shell, and grows to the full size of its 
kind. In the adult state, the female bird possesses an organ 
termed the ovafy, in which nucleated cells, the primitive ova, 
which correspond with the embryo cells of the plant, are 
developed. Each of these grows and becomes invested by 
the materials of the egg ; and, before it is laid, it undergoes 
a process of division, whereby it is converted into the em- 
bryonic patch. Or cicatricula, from which this series of 
changing forms has proceeded. 

Thus the pigeon is developed from a simple nucleated 
cell, by a process of evolution, similar in principle, how- 
ever dissimilar in its results, to that which gives rise to the 
pea-plant. The adult pigeon is an aggregate of modified 
cells, descended by repeated division from the primitive 
egg cell ; and this aggregate assumes a series of successive 



XTV.] LIVING MATTER AND ITS EFFECTS. 227 

forms of graduall}'' increasing complexity. Finally, cells 
are given off and extruded from the body as eggs ; 
each of which is competent to run anew through the 
series, characteristic of that form of living matter known 
as a pigeon. 

Hence, there is a very close analogy between the animal 
and the vegetable forms of life at present under considera- 
tion, but the differences are no less striking. The pigeon 
cannot live on a watery solution of ammoniacal and mineral 
salts, however much fresh air and sunshine may be added to 
this diet. It has no power of manufacturing the protein 
compounds, or the fatty or saccharine matter of its body 
out of simpler substances ; but, directly or indirectly, it 
is dependent upon the plant for all the most important 
constituents of its body. 

Like all other animals, the pigeon is a consumer, not a 
producer. The complex substances, which it obtains from 
the peas on which it feeds, are assimilated to its own sub- 
stance, and are then slowly burned by the oxygen which it 
obtains by the process of respiration. The animal is in 
fact a machine, fed by the materials it derives from the vege- 
table world, as a steam-engine is fed with fuel Like the 
steam-engine, it derives its motor power from combustion ; 
and, as in the case of the steam-engine, the products of the 
combustion are incessantly removed from the machine. 
The smoke and ashes of the animal are the carbonic acid 
evolved in expiration, and the fsecal and urinary excreta. 
The latter are returned to the earth in a more or less fluid, 
or, at any rate, soluble form ; the former is diffused through 
the air. 

When the bird dies, the soft parts rapidly putrefy, and pass 
off, as gaseous and fluid products, into the air and the Avater. 
The dense bones resist decay longer ; but, sooner or later, 

Q 2 



228 PHYSIOGRAPHY. [ckap. 

even the lime salts by which they are hardened are dis- 
solved away ; and the<solid animal fabric returns to swell the 
sum of the fluids and gases from which, through the plant, 
it has been derived. But, under like conditions to those 
which have been mentioned in the case of the plant, the 
bones may be covered up and protected from further decay, 
or may become infiltrated with calcareous or siliceous matters ; 
and thus, as a " fossil bird," a pigeon may form an integral 
part of the solid crust of the earth. 

It will be apparent that pigeons and peas, or more broadly, 
the animal and the plant, respectively represent, in the world 
of life, the destructive and the reparative powers of the not- 
living world — the forces of denudation and of upheaval. 
The animal destroys living matter and the products of its 
activity, and gives back to the earth the elements of which 
such matter is composed, in the form of carbonic acid, am- 
moniacal and mineral salts. The plant, on the contrary, 
builds up living matter, and raises the lifeless into the world 
of life. There is a continual circulation of the matter of the 
siirface of the globe from lifelessness to life, and from life 
back again to lifelessness. 

If pigeons and peas were the only forms of life, the 
balance of solid and fluid constituents of the globe would 
hardly be affected by their existence. Every pigeon and 
every pea, as has been seen, represents a certain amount 
of liquid and gas transmuted into the solid form; but, under 
ordinary circumstances, the solids thus withdrawn, return to 
the fluid and gaseous states within a short time after the 
death of the body which they constitute. It is hardly con- 
ceivable that, under any circumstances, fossil pigeons or 
fossil peas should make a sensible addition to what may, 
at any rate in a relative sense, be termed the permanent 
crust of the earth. 



X!V.] LIVING MATTER AND ITS EFFECTS. 229 

But it is otherwise with plants and animals which live 
under conditions more favourable for preservation ; and, in 
which, the earthy and less perishable constituents enter in 
large proportion into the composition of the body. There 
is much more likelihood that the remains of animals and 
plants which live in the sea, or in rivers, or which haunt 
marshes and lakes, should be fossilized, than that those of 
the dwellers on dry land should be so preserved. And the 
greater the quantity of salts of lime, or of silica, or of other 
slowly soluble ingredients, in the body of an animal or of a 
plant, the longer will its fabric be in disappearing, and the 
greater the chances of its preservation. 

Along the shores of the Isle of Sheppey, in the estuary of 
the Thames, it. is by no means uncommon to find fossils 
which have fallen out of the clay cliffs in the course of their 
destruction by the sea. Many of these fossils are the hard 
fruits of trees, which lived at the time when the clay was in 
course of formation. The fruits appear to have fallen from 
the trees which bore them, and which probably grew on the 
banks of a river ; and then to have been carried down by the 
river to its estuary, where they were embedded in the fine 
mud which has since been hardened into the clay of the 
Sheppey chffs. This is the same kind of clay as that on 
which the metropolis stands— the common London clay. 
The vegetation of this part of the world, at the time repre- 
sented by this clay, must have been very different in character 
from that of the present day. Many of the fruits, for instance, 
are the produce of palm-like trees {Nipa) akin to the screw- 
pines, and similar to those now growing in Bengal, in the 
Philippine Islands, and elsewhere in the East Indian Archi- 
pelago ; while others are the cones of plants {Proteacece) 
similar to those which at the present day flourish in Australia. 
Fig. 62, shows one of these fruits from the London clay of 



230 



PHYSIOGRAPHY. 



[chap. 



Sheppey. It must be borne in mind, that such fossils 
form only an insignificant portion of the bulk of the rocks 
in which they are embedded. There are, however, othei 
organisms which enter so largely into the composition of 
certain deposits as to form by far the greater proportion of 
their bulk. 

Thus, there is a well known substance, which has been 
used in the arts for many years as a polishing material, 
under the name of Tj-ipoli. It is a kind of rottenstone, 
which occurs in large deposits in many parts of the world, 
but is especially abundant at Bilin, in Bohemia, where it 
forms strata of considerable extent, 
one bed measuring as much as four- 
teen feet in thickness. In some 
places, the tripoli is a soft friable 
rock, while, in other localities, it is 
so hard as to be known as " polish- 
ing slate." Chemically, it is almost 
pure silica, like the silica of rock- 
crystal ; but examination under a 
microscope shows, at once, that it 
Fig. 63.— Fossil fruLt(M>a^zV^i is not simply mineral silica. Indeed, 

eUiiticits) from the London , t,,i c .^ • i- ^• • cc 

clay, Sheppey. whcn a little Ol thlS topoh IS suih- 

ciently magnified, it is seen to be 
(nade up, not of shapeless mineral particles, nor of minute 
crystals of silica, but of such beautifully-formed objects as 
those represented in Fig. 64. It was shown, many years 
ago, by the late Prof. Ehrenberg, of Berlin, • that these 
delicate objects in the tripoli are identical with the siliceous 
cases characteristic of a group of minute organisms called 
Diatoms. The diatoms are found living both in salt and 
in fresh water, but the kinds commonly preserved in tripoli 
are characteristic of fresh water, and hence it is concluded 




XIV.] LIVING MATTER AND ITS EFFECTS. 231 

that the material was probably deposited at the bottom of 
lakes or in marshes. 

When a living diatom is examined, it is seen that the 
siliceous case incloses a particle of protoplasm. A diatom 
is, in fact, a simple vegetable cell, but its pecuHarity lies 
in the fact that the cell is able to separate, or secrete^ from 
the surrounding water, that chemical combination which is 
called " silica," and which exists, in minute proportion, dis- 
solved in most natural waters. The silica thus appropriated 
by the diatom forms a solid case, which incloses the proto- 
plasm, and often exhibits a beautifully-sculptured surface. 




Fig 64. — Microscopic section of diatomaceous deposit, Mourne Mountains. 
Ireland. Magnified about i6o diameters. 



On the death of a diatom, the protoplasm decomposes 
and disappears ; but the siUceous shield, although it is very 
slowly dissolved by water, is not easily perishable, and it 
therefore remains as a solid body at the bottom of the water. 
It is true the diatoms are very minute, but they compensate 
for this by their extraordinary abundance. In some 
estuaries, they are so abundant that the accumulation of their 



232 PHYSIOGRAPHY. [chap. 

hard envelopes contributes largely to shallowing the water 
and blocking up harbours. It was estimated by Ehrenberg 
that as much as. 18,000 cubic feet of siliceous organisms were 
accumulated every year in the harbour of Wismar, in the 
Baltic. Sir J. Hooker refers to the enormous multitudes 
of diatoms in the waters and in the ice of the South Polar 
Ocean ; and a deposit, or ooze, consisting principally of their 
siliceous casings, was found along the flanks of the Victoria 
Barrier, extending over an area which measured as much as 
400 miles in length and 200 in breadth. During the voyage 
of the Challenger, a similar diatomaceous ooze was found, 
as a pale straw-coloured deposit, in certain parts of the 
Southern Ocean ; and diatoms are seen in abundance on 
the surface of many seas, especially where fresh water is 
brought down by rivers. Insignificant as diatoms appear 
when regarded individually, it is clear that, in consequence 
of their vast numbers, the rapidity of their multiplication, and 
the resistance of their siliceous cases to destruction, they 
may play a very important part in the formation of certain 
deposits which will eventually constitute siliceous rocks. In 
fact, Ehrenberg has shown that the loose siliceous particles 
of the diatomaceous deposit at Bilin may become altered 
into a compact rock by the percolation of water. This very 
slowly dissolves the silica and then re deposits it, as a hard 
opal-like rock, in which the organic structure is well-nigh 
obliterated. 

There are not many plants that have power, like the 
simple diatoms, to encase their cells in such hard material 
as silica. In the Grasses, however, the cells forming the 
coating of the stalks contain a good deal of silica, which 
confers rigidity upon the structure; and one of the Horse- 
tails^ is so rich in this substance that it is imported from 

' Equisetum hyeinale. 



XIV.] LIVING MATTER AND ITS EFFECTS. 233 

Holland, under the name of " Dutch rushes," for use as a 
polishing material. But, even where there is no special 
deposition of mineral matter in the plant-cells, the walls of 
the cells themselves are commonly formed of a compact 
membrane which may offer considerable firmness of texture. 
The cell-membrane consists of the material called cellulose, 
which difters essentially from the inclosed protoplasm, in 
that it contains no nitrogen, but rather resembles starch in 
chemical composition. In woody plants, the cell walls be- 
come very much thickened ; and the accumulated woody 
matter, which is insoluble in water, contributes to the strength 
and support of the vegetable structure, and decays but slowly. 
Hence, the accumulation of remains of plants may, under 
proper conditions, yield deposits of considerable durability. 

Accumulations of partially decomposed vegetable matter 
form the substance known as peat or turf. This is pro- 
duced only under certain conditions of moisture and tem- 
perature ; damp ground, in a temperate climate, being the 
situation most favourable to its formation. In this part of 
the world, the principal peat-forming plants are certain 
mosses known to botanists under the generic name of 
Sphagnum. The stems of the bog-moss die away in their 
lower part, while the upper portion continues to grow freely. 
The interwoven dead portions form a tangled mass, which 
holds water like a sponge and favours the growth of the 
moss above. Remains of other plants become mixed with 
the mosses, and contribute to the formation of the peat, 
while trunks of trees occasionally get embedded in the 
bog. Muddy matter is likewise washed during floods, 
and helps to consohdate the felted mass and to produce a 
deposit of considerable firmness. The rate at which the 
peat grows varies greatly under different conditions, but 
some notion of the rate may be gained from the fact that 



234 PHYSIOGRAPHY. [chap. 

Roman remains, and even Roman roads, have been found 
beneath eight feet of peat. In Ireland, peat-bogs are so 
abundant as to cover about one-tenth of the entire surface 
of the country; and, in some cases, the peat may be as much 
as forty feet in thickness. The peat is cut from the bog, in 
brick-shaped blocks, by means of a peculiar spade known 
as a " slade," and, after being dried in stacks, is used as fuel. 
In England, peat is not so important as in Ireland, but it is 
to be found in many damp localities. It was mentioned, in 
the last chapter (p. 212), that an old peaty soil extends 
for miles along the estuary of the Thames, though hidden 
beneath the surface. 

In the deeper, and therefore older, parts of a thick peat- 
bog, where the decomposing matter is most compressed and 
altered, it usually takes the form of a brownish-black, slightly 
compact mass, in which the vegetable structure may be 
almost obliterated : the material is in fact converted into a 
substance not altogether unlike coal. Indeed, the resemblance 
has led to the suggestion that beds of coal may, in some 
cases, have been formed by the alteration of old peat-bogs. 
Although there are certain objections to such a view, it is 
nevertheless beyond question that coal owes its origin to 
the alteration of vegetable matter. The evidence upon 
which this conclusion is founded is derived, partly, from the 
chemical and microscopic structure of the coal and, partly, 
from the conditions under which the substance is found in 
nature. 

Coal occurs in the shape of beds, or seams, of variable 
thickness, associated with shales, sandstones, and other sedi- 
mentary rocks. The succession of strata, or measures, cut 
through in a colliery, is generally similar to that represented 
in Fig. 65, but the series may include hundreds of separate 
beds. The "roof" of the coal, or the rock immediately 



XIV.] LIVING MATTER AND ITS EFFECTS. 235 

above the seam, is commonly a shale, which when split into 
layers is very frequently found to inclose impressions of 
plants. Perhaps the commonest of these remains are the 
graceful leaves ox fronds ^ of ferns, which are often strikingly 
similar to those living at the present day. Although, in 
these islands, ferns never attain to the size of trees, yet, in 
countries where the climate is very warm and moist, as in 
Ne\v Zealand, they form trees fifty or sixty feet in height. 



"/)\\^ -'/yix ^'/v^ ^W 



Sandstone. 

Shale. 
Coal. 

Underclay. 
Sandstone. 



^^^^W^^WjW 



Shale. 
'_ Sandstone. 

= Shale. 
Coal. 
Underclay. 

Sandstone. 



Fig. 65. — Section of coal measures. 

Such tree-ferns lived also in this country, at the time of the 
formation of the shales which are found in association with 
the coal. 

In addition to the impressions of plants found in the shales 
above the coal seams, vegetable remains are also met with 
in rocks beneath the coal, forming what is called the " floor." 
It was pointed out many years ago by the late Sir W. Logan, 

■* A. fi'07td differs from an ordinary leaf in usually bearing fructifica- 
tion. The ferns have no flowers, and the fruit is generally developed 
on the frond. 



236 



PHYSIOGRAPHY. 



[chap. 



when examining the great coal-field of South Wales, that 
each bed of coal is supported by a layer of shale known as 
under-day or smt-eafth, as shown in Fig. 65. Whatever the 
number of seams of coal — and in some cases they are very 
numerous — there is always just the same number of under- 
clays. Moreover these underclays usually contain such 
bodies as that figured in Fig. 66, which are never found 
in the roof of the coal. Such objects had long been 
known to geologists under the name of stiginarm^ but 
though it was clear they represented some part of a plant, 
their precise nature long remained an enigma. At length a 




Fig. 66. — Stig)iiariaficoides; a coal measure fossil. 



railway cutting through the Lancashire coal-field exposed 
half a dozen trees resting upon a seam of coal, but sending 
their strong roots downwards into the under- clay, where 
they ramified in all directions and gave off rootlets. It was 
found by Mr, Binney that these roots were nothing but the 
well-known stigmm-ice, and that the characteristic stigmata, or 
pits, were not leaf-scars, as had been suggested, but points 
from which rootlets had been given off. The ~ stigmariae 
passed upwards into fluted tree-stems, which are not un- 

1 Stigmaria, from a-Tly/xa, sligma, "a mark;" in allusion to the 
scars left by the rootlets. 



XIV.] LIVING MATTER AND ITS EFFECTS. 



237 



commonly found in the coal and the shales, and are known 
as sigillaricR (Fig. 67).^ There is consequently no doubt 
that the stigmaria is nothing but the root of the sigillaria, 
and that the under-clay represents the soil of an ancient 
forest in which these and other trees flourished. 

In examining one of these sigillaria-stems, it will pro- 
bably be found that the bulk of the trunk consists of 
stony matter, but that this is coated by a thin layer of 




Fig. 67. — Sigillaria attached to stigmarian roots. 



coal which represents the original bark of the tree. Hence, it 
may be assumed that the old trunk rotted away, leaving 
a hollow tube of bark which has become transformed into 
coal. But, although coal may have beera produced in this 
way to a limited extent, it would be rash to conclude 
that the whole of our coal was formed by such a trans- 
formation. What kinds of vegetable matter have been 
concerned in the production of coal cannot be determined 
without the aid of the microscope. 

On attempting to break a mass of coal, it will generally 
be found that it splits much more readily in certain 

^ Sig^iUaria, from Lat, sigillum, a seal ; the leaf scars resembling the 
impressions of a seal 



238 PHYSIOGRAPHY. [chap. 

directions than in others. Thus, it breaks easily along the 
planes of bedding, which of course njn parallel to the 
general stratification of the coal-measures. The top and 
bottom layers thus broken, commonly present dull black 
almost sooty surfaces, and easily soil the fingers when 
handled. But the mass of coal also splits with ease in 
certain directions which run vertically across the stratifi- 
cation, and the broken surfaces thus produced are generally 
bright and smooth, and do not soil the fingers : the direction 
along which these joints run is often known as the " face " 
of the coal. Then there is a third set of planes at right 




Fig. 68. — Cuboidal block of coal split along natural planes. 

angles to both the other sets, and less perfect, so that the 
fracture here is more irregular ; this direction is sometimes 
called the " end " of the coal. On the whole, then, there 
are three directions, perpendicular to one another, in which 
the coal may be divided ; and it thus yields blocks more or 
less regular in shape, and roughly resembling cubes or dies, 
such as that shown in Fig. 68. 

The dull black substance running along the planes of 
stratification or bedding of a piece of coal is sometimes 
called, from its resemblance to charcoal, mineral charcoal, 
and it is likewise known as mother of coal. This substance 



XIV.] LIVING MATTER AND ITS EFFECTS. 239 

often fibrous, and is made up to a large extent of the 
remains of stems and leaves. But the constitution of the 
mass of the coal is very different from that of the mineral 
charcoal, which indeed forms only thin layers spread out 
between the lamina of the coal. If a slice of coal, cut so 
thin as to be partially transparent, is examined under the 
microscope by light passing through its substance, it will 
usually present some such appearance as that shown in 
Fig. 69.^ This section, which has been taken parallel to the 




Fig. 69. — Microscopic section of Better Bed coal, Yorkshire. Magnified 25 
diameters. 

face of the coal, shows a blackish or dark-brown mass 
forming the ground, in which are embedded numerous 
granules and streaks of yellowish colour. These streaks 
represent the edges of minute bags which have been cut 
through, and which, in certain coals, may be seen entire 
even with the naked eye. Thus, there is a valuable seam of 
coal near Bradford, in Yorkshire, known as the " Better 
Bed," which contains vast numbers of these little discs, each 

1 From a paper by Mr. E. T. Newton in the Geological Magazine, 
Dec. 2, Vol. ii., No. 8, August, 1875. 



240 PHYSIOGRAPHY. [chaP. 

about aV^^ ^^ ^"^ i'^'^^^ ^'"^ diameter, and therefore readily 
visible. These discs are the larger bodies cut through in 
the section, and appear to be sacs which sometimes inclose 
granules similar to those scattered through the dark ground- 
mass, and which are not more perhaps than ^^ of an inch 
in diameter. Botanists conclude that these minute bodies 
are the spores} or reproductive bodies of a flowerless plant; 
while Prof. Morris suggested, many years ago, that the larger 
bodies are the cases which inclosed the spores and are 
themselves known as sporajigia. Similar bodies are well 
seen in microscopic sections of the curious combustible 
substance known as " white coal " which is in course of 
formation in Australia. 

There can be no doubt that these spores and spore- 
cases were shed by trees closely related to those extinct 
forms which are well-known under the x\'dsa.to{ Lepidodendroti?' 
Remains of the lepidodendron have been found, with cones 
still pendant from the branches of the tree ; and similar 
cones, called Lepidosfrobt, are scattered in abundance through 
the coal-bearing rocks. The cones are made up of scales, 
and in some specimens it is possible to detect spore-cases, 
containing spores, still preserved between the scales, 
Mr. Carruthers has given the name of Flemiiigites to a lepi- 
dodendroid plant which has been found to have spores, 
closely resembling those which occur in the mass of the 
coal. There seems therefore scarcely any room for doubt 
that the little bodies so plentifully scattered through most 
coals were derived from plants more or less resembling the 
lepidodendron. 

1 Spores are one of the kinds of bodies by which flowerless plants are 
reproduced. 

^ Lepidodendi'on, from XtTriy, lepis, a scale ; Sei-Spov, dendron, a tree ; 
in allusion to the scale-like leaf-scars on the stems. 



XIV.] LIVING MATTER AND ITS EFFECTS. 241 

But what kind of trees were these ancient inhabitants 
of the coal-forests, and to what existing plants could they 
claim kinship ? To answer this question it is necessary to 
turn not to our forest trees, but to such lowly plants as the 
club-moss or Lycopodiuni. It may seem almost absurd to 
compare such different objects as these; for the club-moss 
is a weak herb, and even in the most favourable conditions 
does not rise to a height of more than two or three feet, 
while the lepidodendron must have been a gigantic tree, 
certainly reaching, in some cases, to a height of a hundred 
feet. Yet, in the form of their stems, and in the character 
of their fructification, the resemblance between the two is so 
striking, that the student is forced to admit that the club- 
moss is but a miniature edition of the old lepidodendron. 
But, though the ancient and the modern plants thus differ so 
much in size, it is curious to note that their spores are of 
nearly the same dimensions.^ 

At first sight, it no doubt seems surprising that objects so 
minute as the spores and spore-cases of extinct plants allied 
to the club-moss should form any considerable proportion 
of those vast masses of coal which occur in beds several 
feet in thickness, and extend over areas measured in miles. 
Yet in this case, as in that of the diatoms, the enormous 
numbers compensate for want of size in the individuals. 
Clouds of yellow dust, consisting of spores, may be 
shaken from a branch of club-moss ; and the spores of the 
diminutive species still living are so abundant that they 
form an article of commerce. The druggist rolls his pills 

^ In some of the living club-mosses there are two kinds of spores, 
one being much larger than the other. The larger are known as inacro- 
spores, whilst the smaller are called mic7-ospores. Prof. Williamson, of 
Manchester, who has paid great attention to the structure of coal-plants, 
has made the important suggestion that the large bodies, termed 
"sporangia" in the text, are really macrospores. 

R 



242 PHYSIOGRAPHY. [chap. 

in lycopodium spores ; and, by thus coating them with a 
resinous powder, enables them to roll over the tongue with- 
out direct contact with its moist surface. And, before the 
days of the electric light, the stage-manager was in the 
habit of using this highly combustible resinous material, 
under the name of " vegetable brimstone," to produce a 
blaze of mimic lightning. 

From what has now been said, it appears probable that 
most coal has been formed in something like the following 
way. A forest of lepidodendrons, sigillari'as, ferns, and other 
plants, grew up on an old land-surface, which is now repre- 
sented by the under-clay, or its equivalent. Season after 
season, showers of spores fell from these flowerless plants, 
and, accumulating on the soil, became mixed with the 
fallen fronds, and with larger or smaller portions of the stems 
of the surrounding trees. While a large proportion of the 
soft vegetable matter slowly disappeared by decay, or left 
only a highly carbonized residue, of which the part that 
retains a recognizable structure is the " mother of coal," the 
resinous spores resisted decomposition, and remain dis- 
tinguishable in all the less altered coals. The roots of the 
LepidodeJidra were often preserved by the clay in which 
they grew, and became the fossil stig??iarice. 

When a layer of vegetable soil had thus accumulated 
•"O a considerable thickness, the land slowly subsided, and 
the old forest was buried beneath deposits of mud and 
sand, w^hich have since hardened into shales and sand- 
stones. Compressed beneath these sediments, the vege- 
table matter underwent peculiar changes, which resulted 
m the formation of coal. Then a time came when the 
sedimentary deposits were upheaved, and another forest 
sprang up on the new land, forming a second bed of 
coal. Hence every seara of coal indicates fresh movement 



XIV.] LIVING MATTER AND ITS EFFECTS. 243 

of the ground ; and, when it is remembered that, in the 
South Wales coal-field, as many as eighty distinct beds of 
coal may be recognized, it will be seen that the coal- 
measures offer striking evidence of oscillations of the level 
of the land. Between each elevation and depression, there 
must have been time enough for the formation of a thick 
vegetable soil, and, in some cases, this must have taken vast 
periods of time; thus, in South Staffordshire, there is, or 
rather was, a famous bed of coal measuring as much as 
thi^iy feet in thickness. Remembering, then, the slow growth 
of a forest, the great thickness of some of our coal seams, 
and the number of separate beds in the coal-measures, it 
will be readily conceded that these strata represent a lapse 
of time which is probably to be counted by hundreds of 
thousands of years. 

Before it was understood that each bed of coal grew 
where it now stands, it was supposed, by many geologists, 
that coal had been formed by the alteration of wood 
drifted out to sea. Great rafts of timber and other ac- 
cumulations of vegetable matter are unquestionably carried 
down by such a river as the Mississippi; and this matter, 
becoming buried in the silt of the estuary, might undergo 
changes resulting in the formation of coal. But, though 
small deposits of coal may have been formed in this way, 
no accumulation of drii't-wood would be competent to pro- 
duce beds of pure coal of uniform thickness and great 
extension, such as those in any of our coal-fields. More- 
over, there are the stlgmarm to show that the plants grew 
where their remains are found. 

There is, however, a kind of imperfect coal which shows 
by its texture that it has been formed from wood. So 
ligneous indeed is its texture that the coal is commonly 
called lignite. In this country, such coal is found only in 

R 2 



244 PHYSIOGRAPHY. [chap 

insignificant quantity at Bovey Tracey in Devonshire, but 
in many countries, which are poor in true coal, the b'gnite 
occurs in large deposits and is an important fuel. A few 
years ago, some old timbering in a mine in the Hartz, 
known to date back about four hundred years, was found to 
be converted into this lignite or "brown coal." Hence, 
there can be no doubt that, under certain conditions of 
decay, wood may be transformed into a coal-like material. 
Lignite may be regarded as vegetable matter not com- 
pletely mineralized. Even the ordinary coal of our own 
country is subject to further alteration, and may become 
still more removed in characters from its original condition. 
Thus, in the coal-field of Sotith Wales, a curious change may 
be traced in passing from one end of the field to the other. 
In the eastern part, the coal is of the ordinary kind, such as 
is seen in our scuttles, and is called bitummoiis coal ; towards 
the middle of the field this passes into semi-bituminous coal, 
which is a kind of fuel that does not burn with a bright 
.jassy flame, but is valued for feeding the engines of steamers, 
since it emits but little smoke \ finally, in the western part of 
the coal-field, this steam-coal passes into a substance called 
anthracite^ which is still less inflammable and still more 
removed from the original form of vegetable matter. Such 
changes in the character of the coal appear to be closely con- 
nected with the disturbance and fracture of the coal seams. 
Most deposits of coal have been thrown into basin-like 
shapes, and the beds are often broken up and otherwist 
disturbed by, the injection of igneous rocks and by lateral 
pressure. In these disturbed regions, the coal has been 
altered into an anthracitic material. The alteration which 
has been effected appears to be much like that which 
takes place when coal is artificially distilled, for the 
1 AnihrtKite, from &vdoal, anthrax coal or charcoaL 



XIV.] 



LIVING MATTER AND ITS EFFECTS. 



245 



manufacture of ordinary illuminating gas. That part of 
the coal which contributes to the blaze of a fire is removed, 
while the coke-like portion is left behind. 

The chemical changes which have taken place during the 
conversion of vegetable matter into the several varieties of 
coal may be seen by a comparison of their analyses, as 
given in the following table ; — 



Wood (Oali) ..... 

Peat (Irish) 

IJgnite (Bovey Tracey) . 
Bituminous Coal (Newcastle) 
Steam Coal (South Wales) . 
Anthracite (South Wales) 



Carbon. 
48-94 
55-62 
69-94 
88-42 
92-10 
94 '05 



Hydro- Oxygen and 
gen. Niirogen.' 

45'!^ 



5 '94 
6-8S 

5-95 
5-61 
5-28 
3-38 



37'5o 
24- n- 

5 97 
2-62 

2-57 



Changes of the kind indicated by these analyses have 
been carried on, during the past history of the earth, on an 
enormous scale ; and the great extent and thickness of the 
deposits of coal, which have thus been produced, show 
that vegetable life has played no insignificant part in the 
formation of the rock-masses which build up the earth's 
crust. 



' The quantity of nitrogen being small is added to the oxygen. 
analyses are calculated exclusively of the ash, or mineral matter 
tained in the coals. 

^ Exclusive of nitrogen. 



The 



CHAPTER XV. 

THE FORMATION OF LAND BY ANIMAL AGENCIES CORAL 

LAND. 

It has already been pointed out that when an aquatic 
animal dies, its hard parts, such as a shell, or bones, if it 
happen to possess any, will stand a fair chance of consti- 
tuting a permanent contribution to the solid materials of 
the earth, by becoming embedded in mud, and in this way 
preserved from destruction. 

Such names as "Shell-haven," near Tilbury, on the Essex 
coast, and " Shell-ness," in the Isle of Sheppey, sufficiently 
indicate the abundance of shells, which accumulate in certain 
regions of the estuary of the Thames ; and, on many other 
parts of the English coast, enormous multitudes of shells are 
scattered upon the sea-beach and embedded in its sands 
and mud. 

Vast quantities of dead shells accumulate on oyster-beds^ 
and the dredge brings up similar objects, wherever it is 
allowed to scrape along the bottom of the sea, around oui 
coasts. Moreover, in some parts of the Channel, small 
reefs are built up of nothing but the sandy habitations which 
are fabricated by certain marine worms. 

This operation of the formation of new land by ani 
tnal agents is manifested in the most conspicuous 



CH. XV.] CORAL LAND. • 247 

manner and on a gigantic scale, by the coral reefs and 
islands of which we hear so much in accounts of voyages 
in tropical seas. 

It is extremely common to hear, or read, that these 
m-asses of land are constructed by coral " insects." As a 
matter of fact, however, the animals mainly concerned in 
the formation of these deposits are widely different from 
insects ; while they are very similar to certain marine 
organisms, of much simpler structure than any insects, which 
abound on our own coasts. 

Scarcely any visitor to the sea-side can fail to be familiar 
with the peculiar flower-like creatures which are popularly 
called sea anemones} They are commonly to be found 
attached to the rocks in little pools of salt water left 
between tides. The body of the sea anemone is a fleshy 
sac, more or less cylindrical in shape, and closed at 
one end, which forms the base, by means of which the 
creature fixes itself to any solid object. Upon occasion, it 
can quit its hold and, by movement of this fleshy base, is 
able to crawl over the sea-bottom. In marine aquaria, 
sea- anemones may sometimes be seen creeping up the 
glass sides of the tank, in this fashion. At the opposite end 
of the cylindrical body, there is a mouth, surrounded by a 
great number of feelers or tentacles, disposed in a circle, or 
more commonly in several circles one within another. So 
sensitive are these feelers that, if one be lightly touched, 
they are all quickly drawn in, and the creature shrinks to 
a small conical mass, looking like a mere knob of jelly 
stuck to a stone. But, when the feelers are freely spread out, 
they form a graceful crown, variously coloured and giving 
the animal a very flower-like appearance, not altogether 

^ Anemone, from the flower so called ; from aue/jLos [anemos) wind, in 
allusion to the flower being ersily blown about by the breeze. 



248 PHYSIOGRAPHY. [chap. 

unlike that of a China-aster, or some other member of 
that great group of plants represented by our daisies and 
dandelions. 

If any little animal, such as a shrimp, chance to come 
within reach of the outspread feelers, it is at once carried to 
the mouth and thrust into a sac, which occupies the centre of 
the body. Between the walls of this sac and those of the 
body, there is a wide space, so that the arrangement may be 
compared to that of a common inkstand; the inner sack repre- 
senting the glass vessel which holds the ink, and the rest of 
the body the body of the inkstand, into which the ink-holder 
drops. And, just as there are holes round the top of the 
inkstand for holding pens, which holes open into the inter- 
space between the inkholder and the body of the inkstand ; 
so, round the top of the body of the sea-anemone, there are 
openings, by which the cavities contained within the feelers 
communicate with the interspace between the inner and the 
outer sacs. Beyond this point, however, there are two 
important differences between the sea-anemone and the 
inkstand. For the inner sac is open at the bottom ; and, con- 
sequently, the interspace between the inner and the outer 
sacs, and the cavities of the feelers, are in free communication 
with the cavity of the inner sac ; and, therefore, by means of 
the mouth, with the exterior. Hence, all the cavities are 
full of sea-water. In the second place, in the sea-anemone, 
a number of vertical partitions stretch from the inner sac to 
the outer wall of the body, whereby the interspace between 
the two is divided into numerous chambers. 

The food which is taken into the inner sac undergoes 
digestion ; its nutritive parts are dissolved and are 
diffused through the fluid which fills the body, and which 
thus serves the purpose of blood ; while the indigestible 
hard parts are cast out again by the mouth. The body oi 



XV.] CORAL LAND. 249 

a true insect is divided into segments ; it has a digestive canal 
which does not open into the cavity of the body ; it has 
distinct organs of circulation and respiration, and a peculiarly 
formed nervous system. None of these features are to be 
met with in the sea-anemone ; which, therefore, is an animal 
of much lower grade than an insect. Indeed, it is much 
more nearly allied to the jelly-fishes, which float in the sea, 
and to the fresh-water polypes of our ponds. The general 
name of "polype"^ is, in fact, applied to the sea-anemones 
no less than to the latter. 

The substance of the body of the common sea- 
anemones is quite soft, and none of them acquire a greater 
consistency than that of a piece of leather. But there are a 
few animals, which live at considerable depths in our own 
seas, and very many in other parts of the ocean, the 
structure of which is, in all essential respects, similar to 
that of the sea-anemones, but which nevertheless possess a 
very hard skeleton (Fig. 70). This skeletor inasmuch as 
it is formed by the solidification of the base and side-walls 
of the body of the polype, necessarily has the form of a cup, 
and it is termed a cup coral, to distinguish it from other 
kinds of coral such as the red coral, which, though produced 
by similar animals, are formed in a different manner. Not 
only are the walls of the body thus hardened, but vertical 
partitions of the same character extend from the walls of 
the cup to its centre, in correspondence with the partitions 
which divide the cavity between the inner sac and the body- 
wall. The hardening of the lower part of the body of the 
coral polype, and that of the partitioiis, is effected by the 
deposition, within their substance, of carbonate of lime 

^ Polype, hova TToXvs, pohis, "many," and ttovs, pous, "foot;" an 
animal with several feet or several tentacles ; thus the octopus derives 
it- name from having eight of these organs. 



250 



PHYSIOGRAPHY. 



[chap. 



extracted from the sea-water in which the animal Hves ; just 
as the calcareous salts of the bones are extracted from the 
milk, and deposited in those parts of the body which are 
becoming bone, in a growing infant. The deposit converts 
the base of the polype into a solid cement, which fixes 
the animal to the surface to which it is attached ; and, if 
the polype goes on growing not merely in height, but in 
breadth, while the process of calcification extends as it 




Fig. 70. — Caryophyllia Smiihii, a coral-polype from the coast of Devonshire.' 



grows, the coral will necessarily assume the conical form 
exhibited in Fig. 71. It will be understood that the deposit 
of calcareous matter does not extend into the region of 
the feelers, or into the inner sac, so that the formation 
of the coral skeleton no more interferes with the per- 
formance of the functions of the body in the polype, than 

^ From Mr. Gosse's NaturalisCs Rambles on the Devonshire Coast, 
1853- 



XV.] 



CORAL LAND. 



251 



the development of the bones of a man does with his 
eating and drinking. 

Sooner or later, the coral polype dies ; then, the feelers, 
inner sac, and all the soft upper parts of the body, and 
those which cover the skeleton, decay, and are washed away, 
while the skeleton, or coralhim, as it is called, is left as a 
contribution to the solid floor of the sea (Fig. 72). 

Such soHtary coral polypes as have been described give 




Fig. 71. — Diagrammatic secti. n of a single cup-coral, to sliow the general structure 
of the polype, and the relation of the skeleton to the soft parts. 

a, mouth ; b, inner sac or stomach ; a', its inner opening ; c, the soft outer wall of 
the body ; d, the interspace between the inner sac and the brdy-wall, with its 
partitions ; e, the tentacles ; /, the calcified body-wall or cup of the coral ; g, the 
hard partition of the coral ; k, the base by which the coral is fixed. 



rise to numerous eggs, the young developed from which 
float away, and sooner or later, fixing themselves, take on 
the form of the parent. Very often they have other modes 
of multiplication. A coral polype may give off small 
buds, each of which grows into a perfect animal with its 
own stomach, mouth, and feelers, but remains closely con- 
nected with the parent. In other cases, the coral animal 
spontaneously splits into two halves ; and these, in turn, may 



252 



PHYSIOGRAPHY. 



[chap. 



divide and subdivide, the product of each division growing 
into a perfect polype. By frequent repetition of these 
processes of budding and sphtting, the corals may form 
masses of great size ; in some cases, branching like a tree, 
with separate polypes budding out in all directions ; and, in 
other cases, spreading into a confused mass, like the well 
known "brain-stone coral," which is to be seen in every 
museum. Since the multiplication of the polypes may go 
on ♦o an almost unlimited extent, it is evident that the 




Fig. 72. — Thecopsaiitjuia socialis, Pourtales. 

aggregated mass of coral maybe of enormous size, although 
the separate polypes are but small. In fact, it is the growth 
of coral, in this manner, that forms those masses of land 
which are known as "coral-reefs" and "coral-islands." 

Such land is popularly said to be " built " by the coral- 
animals, but it should be understood that it is not a construc- 
tive work, like the nest of a bird, or the comb of the bee. 
The land is simply an accumulation of the calcareous re- 
mains, or skeletons, of the coral-polypes. The formation of 
this land is, indeed, very much like the formation of the 



XV.] CORAL LAND. • 253 

peat-bog, described in the last chapter. It was there 
shown that the bog-moss dies below, while it continues to 
grow above ; and, in like manner, the coral-polypes die 
below, leaving their calcareous skeletons, while they con- 
tinue budding and growing above : hence, a coral-island 
can be said to be " built " by the polypes only in the same 
sense that a peat moss can be said to be " built " by the 
plants of the remains of which it consists. 

Many islands in tropical seas are skirted by low banks of 
coral -formed rock. At high tide, the surface of the rock 
is, for the most part, submerged, and its position is then 
marked only by a white line of heavy breakers. But, at 
low water, the surface is more or less exposed, forming a 
broad and bare platform, which rises slightly above sea-level. 
Some islands are completely bordered by a margin of this 
coral-rock, while others are fringed only at certain points. 
Where a stream runs down from the land, and carries 
sediment to sea, the reef is generally absent, for the coral- 
polypes do not thrive in muddy water. Rocky ridges, 
which fringe a shore in the manner just described, are known 
as frtnging-7'eefs. 

In other cases, the coral-rock is not directly attached to 
the coast, but stands off at some distance, so as to form a 
barrier, perhaps many miles from land. Such reefs are 
consequently called barrier-reefs. Between the coast and 
the reef there is a channel of comparatively shallow water, 
forming a harbour, to which entrance is gained by a breach 
here and there in the reef, the reef itself constituting a 
natural breakwater. Patches of coral-rock, forming small 
isolated reefs, may be scattered about the quiet channel, 
and the barrier itself may be broken up into a chain of 
detached reefs. Along the north-eastern coast of Australia 
there is a chain of these barrier-reefs stretching for a length 



254 PHYSIOGRAPHY. [chap. 

of about 1,200 miles, and standing at an average distance 
of twenty or thirty miles from the coast. The channel 
between this barrier-reef and the land is termed " the inner 
passage," and has a depth of about twenty or five-and-twenty 
fathoms ; while, outside the barrier-reef, the depth of the sea 
suddenly increases to many hundred fathoms. 

In addition to the fringing and barrier-reefs, there is yet 
another kind, differing from those principally in being 
quite isolated from other land. The coral-rock thus forms 
a true island, rising from the sea usually as a low strip of 
land, more or less ring-shaped, but generally of irregular 
outline. In places, the strip of coral-land may bear a rich 
growth of cocoa-nut palms and other tropical forms of 
vegetation ; while, inside the rim of land, there is a shallow 
lake, or lagoon, of clear green water, which strikingly con- 
trasts with the dazzling white coral-rock of the beach. 
Access to the lagoon is gained by a gap in the shore, and 
thus the island generally presents a horse-shoe shape. 
Several openings may occur in the belt of land, and the 
island consequently becomes broken up into a chain of 
islets. These coral islands are plentifully scattered through 
the Pacific and Indian Oceans, and are often known under 
the Maldive name of atolls. 

In explaining the formation of coral-land, it should be 
remembered that the corals themselves are powerless to 
raise the land above low-water mark, for the polypes perish 
when exposed above water. Dry land is, however, formed 
mechanically ; blocks of dead coral being broken off by 
waves from one part of the rock, and piled up upon 
another. The loose blocks are cemented into compact 
masses by means of coral-sand and coral-mud, produced by 
the tear and wear of the coral-rock. In the case of fringing 
reefs, the seaward, and, in that of atolls, the windward, side 



XV.] CORAL LAND. 255 

of the mass of coral is usually the highest, for it is here that 
the coral-polypes flourish most luxuriantly ; while the dash 
of the breakers, during storms, tears off fragments of the 
coral rock, and heaps them up on this side. It should be 
borne in mind that the land is not entirely formed of corals, 
since other creatures living in the lagoon, and on the banks 
of the reef, contribute their remains to swell the mass. 
Vegetable life too is not without its effect on the formatiop 
of the new land ; and, indeed, the outer edge of a reef is 
often formed, in large measure, of mdlipores, a kind of sea- 
weed, the tissues of which are strongly impregnated with 
carbonate of lime. 

Although corals of some kinds may be found in almost all 
seas, those particular species which grow together in masses, 
and thus form reefs and islands, are limited to the warmer 
parts of the world. Prof. Dana, who has had ample 
opportunity of observation, believes that the reef-forming 
coral-animals are restricted to waters in which the mean tem- 
perature for the month, even in the coldest season, never 
falls below 68° F.^ If, then, a line be drawn through all 
parts of the ocean north of the equator, where tlie coldest 
month has this average temperature, and a similar line south 
of the equator, they will include a zone within which all the 
coral-reefs of the world are situated. It need hardly be said 
that these lines will not be straight lines running in circles, 
round the world, like parallels of latitude, but will be irre- 
gular lines, rising in one part and falling in another, accord- 
ing as the temperature is locally affected by the presence of 
ocean-currents or by the proximity of land. This belt of 
warm water, congenial to the coral-makers, never extends 
beyond about 30 degrees from the equator. 

Though the reef-building corals abound in many parts of 

' Corals and Coral Islands. By James D. Dana, LL.D. 1875. 



256 PHYSIOGRAPHY. [chap. 

this zone, they are not found in all parts of it. They 
are absent, for example, on the western coasts of Africa 
and America ; and, where great rivers debouch, the sedi- 
ment and the fresh water, which they pour into the sea, 
interfere with the growth of the coral polypes. Moreover, 
reef-forming corals are restricted, not only in their super- 
ficial distribution, so as to be limited to certain latitudes, 
but also in their vertical distribution, so as to be limited 
to certain depths. Indeed, the needful conditions for the 
growth of the polypes are found only in comparatively 
shallow water. From the observation? of Mr. Darwin, it 
appears that these corals do not flourish at greater depths 
than between 20 and 30 fathoms, and are, for the most part, 
restricted to about 15 fathoms of water. Knowing this, it 
might not unnaturally be assumed that coral-reefs and coral- 
islands would always be confined to shallow seas. As a 
matter of fact, however, soundings outside a barrier-reef, 
or an atoll, often show an enormous depth of water, the 
outer edge sinking down abruptly like a coral-wall. The 
early navigators knew that coral-islands were not unfrequently 
surrounded by very deep water ; but this fact presented no 
difficulty, until naturalists became aware of the small vertical 
range to which the living corals are limited. Various 
attempts were then made to reconcile the two apparently 
opposed facts ; but no satisfactory explanation was given 
until Mr. Darwin, about forty years ago (in 1837) advanced 
a most ingenious hypothesis, which not only perfectly solved 
the puzzle, but brought the several classes of coral-reefs into 
close relation with each other. 

According to Mr. Darwin's view, the coral-rock has, in all 
cases, been originally formed in water not deeper than about 
20 fathoms; and, when found at greater depths, it must have 
been carried down by subsidence of the rocky foundation 



XV.] CORAL LAND. 257 

on which the polypes lived and died. The details of 
so simple, yet complete an explanation, deserve closer 
examination. 

It has already been shown that coral-pol}T)es can multiply 
by processes of budding and splitting; but it should be 
added, that they can also multiply by means of germs, which 
are thrown off from the parent as free-swimming bodies. 
Suppose that some of these embryonic corals settle upon a 
sloping shore, in shallow water, where the conditions of life 
are favourable ; there they may go on multiplying, until they 
form masses of considerable extent, skirting the land, but 
never extending seawards to a depth of more than 20 or 30 
fathoms. Let the land, with its little fringing-reef, now 
slowly sink ; that part which is carried down lower than 
about 30 fathoms will consist of nothing but dead coral; but 
the upper part of the reef will continue to grow, and, if the 
subsidence be not more rapid than the upward growth, the 
level of the reef will appear to remain stationary, at about the 
sea-level. It has been said that the coral-polype flourishes 
best at the outer margin of the reef, where it is bathed 
by the surf. For this and other reasons, the reef is 
highest at this edge ; while, between t.he outer margin of the 
reef and the shore, there is a channel formed by the entry 
of sea-water during subsidence. In fact, the fringing-reef, 
as it has been slowly carried down, has become converted 
into a barrier-reef. This will be easily understood by re- 
ference to the sections in Figs. 73 and 74. In Fig. 73 an 
island. A, is skirted by a fringing-reef, B B : on the sinking 
of the land to a lower level, as in Fig. 74, the bank of 
coral, B B, becomes thicker by upward growth, and a 
channel, C C, comes to be formed between the barrier 
and the shore. 

Outside the barrier, on its seaward edge, there may be a 

s 



258 



PHYSIOGRAPHY. 



[chap. 




A 



<^X, B 



W^M^yy //^ yM fAJJJyf^i^yyy //m/yyy^^/y(yyyyyy^y:Wym^^//4 

F,Q. y3. — Diagrninnntic section of an island surrounded by a fnnging-reet. 





FivJ. 74. — Diagrammatic section of an island surrounded by a barrier-reef, with 
intervening lagoon. 




Fig. 7s. — Diagrammatic section of a coral island, or atoll, with central lagoon. 



XV.] CORAL LAND. 259 

great depth of water, varying according to the extent to 
which subsidence has gone on. By continued subsidence 
of an island encircled by a barrier, the lagoon, C C, becomes 
wider and wider. At length, only a few rocks may stand 
up in the centre of the lake ; even these may at last dis- 
appear, leaving nothing but a sheet of water surrounded by 
the reef, and the barrier in this way becomes converted into 
an atoll, as shown in Fig. 75. Here the original land, A, has 
entirely disappeared beneath the growth of coral, B B, which 
surrounds the lagoon, C. 

Assuming then that, where barrier-reefs and coral-islands 
occur, they indicate areas of subsidence, Mr. Darwin has 
been able to map out the Pacific and Indian oceans into 
zones in which the land is, or has been, slowly sinking.^ 
These zones alternate with areas in which elevation is pro- 
bably going on, as indicated by the occurrence of active 
volcanoes. Fringing-reefs tell less about movements of the 
sea-bottom, for they may occur where the land is either 
stationary or rising. In some cases, an old fringing-reef is 
found standing high and dry above water, like a raised 
beach, and thus showing clearly that the land has been 
subject to elevation. 

^ 77?,? Structure and Distribution of Coral Ree/s. By Cliarles Darwin, 
M.A., F.R.S. Second Edition. 1874. 



S 2 



CHAPTER XVI. 

THE FORMATION OF LAND BY ANIMAL AGENCIES. — 
FORAMINIFERAL LAND. 

The operations of the reef-building coral polypes, described 

in the last chapter, are carried out on a gigantic scale. The 

Australian barrier-reef, alone, spreads a constantly increasing 

deposit of coral limestone over an area larger than that of 

Scotland ; ^ while the totality of the surface, over which coral 

reefs are spread in the Pacific ocean, exceeds that of Asia, 

Moreover, reefs and atolls are conspicuous objects, forcing 

themselves on the attention of the traveller by their beauty 

and their singularity, and awakening that of the navigator 

by the dangers which they create. But the conversion of the 

contents of the ocean into solid rock is constantly taking 

place, over a still greater area, and probably as rapidly, by 

agents which are inconspicuous, and, indeed, for the most 

part, invisible ; not only by reason of their minuteness, but 

because the results of their work accumulate, not in shallow 

waters, but at the bottom of the deep sea. Out of sight, 

they would be also out of mind, if various circumstances had 

not, of late years, led to the careful exploration of the 

depths of the ocean. 

1 The area of the barrier-reef is estimated to be 33,000 square miles ; 
that of Scotland is 31,324 square miles. 



CH. XVI.] FORAMINIFERAL LAND. ' 261 

Almost everything that is known about the deep-sea bottom 
and its inhabitants has been learnt within the Jast quarter of a 
century. Wlien it was first proposed to bring the Old World 
into relation with the New, by means of a telegraph-cable, it 
became necessary to make a careful survey of that portion 
of the sea-bed on which the cable was to rest. The bed of 
the North Atlantic was first examined, in detail, in 1853, by 
Lieutenant Berryman, in the United States brig Dolphin ; 
and, m 1857, it was thoroughly surveyed, between Ireland 
and Newfoundland, by Captain Dayman in H.Al.S. Cyclops. 
During these surveys, numerous samples of the sea-bottom 
were procured ; and those which were brought up by the 
American survey were submitted to Ehrenberg and Bailey, 
while those from the English survey were examined by 
myself In subsequent years, the exploration of tlie sea- 
bottom has been actively carried on in various parts of the 
world ; and the valuable series of observations made during 
the expedition of H.M.S. Challejiger has given us exac^t 
information of its nature, at a series of stations, in all the 
great oceans. 

In the ordinary method of sounding, or ascertaining the 
depth of the sea, a leaden weight is attached to the end of a 
graduated line, and rapidly run out until the weight strikes 
the bottom. To procure a sample of this bottom, the lead 
IS "armed ; " that is to say, the bottom of the weight, which 
should be slightly hollow, is covered with tallow, and a small 
quantity of the mud or other material at the bottom sticks 
to this grease, and may thus be brought up for examination. 
Such rude means are sufficient for sounding in shallow 
water, but more complicated instruments are required for 
deep-sea soundings. Most of these instruments act upon a 
pnnciple first suggested by Lieutenant Brooke, of the U.S. 
Navy— that of causing the weight to detach itself from the 



262 



PHYSIOGRAPHY. 



[chap. 



line on reaching the bottom. The sounding-Hne thus runs 
down carrying a weight, but comes up free, bringing only a 
sample of the bottom, which is collected on the floor of the 
sea in a cup, or a tube, or a scoop. 

Without referring to the various modifications of sounding 
apparatus which have been employed by successive deep-sea 
expeditions,^ it will be sufficient to describe the sounding 
machine which was very largely used, during the recent 
voyage of the Challetiger. This 
is represented in Fig. 76, and a 
section is given in Fig. 77. The 
apparatus is a modification of one 
which had been used by Captain 
Shortland in H.M.S. Hydra, 
whence it is sometimes called 
the " Hydra Machine ; " its pre- 
sent form, however, is due to 
Lieutenant Baillie. 

This apparatus consists of a 
metal tube a, mostly of iron, five 
and-a-half feet in length, and two 
and-a-half inches in diameter. 
Its upper end is furnished with 
a brass cylinder, b, in which a 
heavy piece of iron works up 
and down, like a piston in a cylin- 
der. At c, this iron is furnished 
with a shoulder, which carries the iron-wire sling to which 
the sinking-weights are attached. These sinkers, d, are made 
of iron, cast in the form of cylinders, each with a central hole; 
they are provided with teeth and notches, so as to fit one 

1 Descriptions and figures of tliese instruments will be found in The 
Depths of the Sea, by Sir C. Wyville Thomson. The sounding apparatus 
itself may be seen in the Museum at South Kensington. 



Fig. 



Fig. 77. 



Deep-sea sounding apparatus used 
by the Challenger. 



xvi.l FORAMINIFERAL LAND. 263 

into another; and thus several may be fitted together, 
forming a compact mass perforated by a central canal, 
through which the tube passes. As the instrument runs 
down, water enters the tube a, at its open end e, and passes 
out through holes in the upper part. On striking the 
bottom, the tube sinks into the mud or other material, and 
a small quantity enters ; and this is prevented from escaping 
by means of a pair of butterfly-valves, opening inwards, 
which are attached to the bottom, e. When the floor of the 
sea is touched, the brass cylinder, b, is pushed up, and 
striking the shoulder, c, of the iron piston, throws off the 
sling, and thus releases the weight. Thus, when the line to 
which the instrument is attached is hauled in, it comes to 
the surface, carrying nothing but the tube full of the sea- 
bottom. It is by means of such instruments that the deep 
sea has been sounded, and samples of the bottom brought 
to the surface for scientific examination. 

The careful soundings made during these surveys revealed 
the remarkable configuration of the Atlantic sea-bed. This 
is shown in Fig. 78, which shows the contour of the floor of 
the sea between Valentia Island off Ireland, and St. John's in 
Newfoundland. It will be seen that there is a gradual down- 
ward slope from the Irish coast, for a distance of about 200 
miles ; then there is a more rapid descent^ leading to a vast 
undulating plain which stretches across the ocean, until it 
reaches a distance of about 300 miles from Newfoundland, 
and from thence it gradually ascends towards the American 
coast. This great submarine plain, which has been called the 

^ In the diagram this descent looks like a steep cliff. But this is a 
deception arising from the exaggeration of the vertical height. Drawn 
to a true scale as in D, Fig. 78, the inclination of the slope is seen to 
be not more than i in 25, or that of a hill of moderate steepness. If 
it were a mere question of gradients, a waggon could be driven along 
the sea-bottom from Ireland to Newfoundland without any difficulty. 



264 



PHYSIOGRAPHY. 



[chap. 



< 



i 



til 

"it'? 



XVI. J 



FORAMINIFERAL LAND. 



261; 



" telegraph plateau," has a width of more than 1000 miles, and 
an average depth of more than 1000 fathoms. It is almost 
uniformly covered with a wide-spread deposit of fine creamy 
or greyish mud, generally called " ooze." When this mud 
is dried, it hardens into a grey friable substance, which may 
be used for writing on a board, as chalk is used. Moreovei, 
when any acid is poured upon the mud, the greater part of it 




Fig. 79 — A. Glohtgerina bnlloides, D'Orb. ; B. Orbulina nniversa, D'Orb. ; C. 
A coccosphere ; D. A coccolith, profile and three-quarters view (after Haeckel, 
much more highly magnified). 

dissolves with effervescence, just as a piece of chalk would 
do ; and it can readily be ascertained that the ooze, like 
chalk, consists mainly of carbonate of lime. 

This calcareous ooze, however, is not mere mineral 
matter; for, when a little is placed under the microscope, 
the greater part of it is seen to consist of such bodies as 
those represented in Fig. 79 A. Each of these consists of 
several globular chambers, one of which is smallest and one 



266 PHYSIOGRAPHY. lChap. 

largest, while the others are of intermediate dimensions, 
disposed around a common centre, and adherent together. 
Each chamber has an opening in the face which is turned 
towards the centre; and, in the living state, all the chambers 
are filled witli a protoplasmic substance, which spreads over 
the surfaces of the chambers, and sends out long radiating 
contractile threads. The walls of the chambers are hard 
and brittle, from the large quantity of carbonate of lime 
which they contain ; and, in the smaller chambers, they are 
very thin and quite transparent. In the larger, they become 
thick, and the outer part of their substance acquires a 
prismatic structure. In specimens taken from the sea with 
great care, the outer surfaces of the chambers are beset with 
long slender calcareous processes, like threads of glass ; but 
these very readily break off. 

The bodies thus described are the skeletons of animals of a 
very simple character, known as Globigerina bidloides, belong- 
ing to the group which has been named the Foramifiifera^ in 
consequence of the numerous perforations usually visible in 
their hard parts. It has been a question whether the 
GlobigerincB live and die at the bottom of the sea, where 
their skeletons are found ; or, whether they live at the 
surface, and the shells in the ooze are therefore merely the 
skeletons of those v/hich have died at the surface, and 
have thence fallen to the bottom. The investigations of 
the Challenger have now placed it beyond doubt that, 
whether any of them live it the bottom or not, they cer- 
tainly swarm in prodigious numbers at, and a few fathoms 
below, the surface. They were taken by the tow-net, in 
all latitudes, over an area extending for between 50° and 60° 
on both sides of the equator ; and though they abounded 
most in warm and temperate climates, tliey were not entirely 
>■ Foraminifera, Lat. foramen, an aperture ; and fero, I carry. 



XVI.] FORAMINIFERAL LAND. 267 

absent towards the northern or the southern limits of this 
range. 

Over the whole of this enormous extent of the ocean, 
therefore, we must imagine an incessant rain of Globigerina 
shells ; which, after falling from the surface, through perhaps 
two or three miles of sea-water, at length rest in, and add to, 
the ooze at the bottom. It is probably an over-estimate if 
we assume that the average bulk of the calcareous matter 
contained in each full grown Globigerhia amounts to 
J -g-g-g^Q-g^th of a cubic inch. Nevertheless, the example of 
the effect of pluvial denudation, however slow and insig- 
nificant the wear and tear of rain and rivers on dry land 
may appear to be, when continued through long ages, in 
destroying the solids of the globe, prepares the mind to 
view, in this incessant downpour of lime-drizzle, a no less 
potent agent of reconstruction. If we suppose that the 
total thickness of the deposit of solid matter on the sea- 
bottom, arising from the foraminiferal shower, is as much as 
one-tenth of an inch a year; then, if the present state of the 
Atlantic and Pacific oceans has existed for only 100,000 
years, this apparently unimportant operation will have 
sufficed to cover their floors with a bed of limestone no 
less than eight hundred feet thick. 

Although the Globigerina shells constitute the greater 
part of the substance of the ooze, the remains of other or- 
ganisms are found with them. Among these, other Foramini- 
fera are very common ; and especially one form, the Orbiilina, 
Fig. 79 B, which is very closely allied to, if not a condition 
of, the Globigerina itself 

Besides these, there are innumerable multitudes of very 
minute saucer-shaped disks, termed coccoiiths, which are 
frequently met with associated together into spheroidal 
aggregations, the coccospheres of Wallich, Fig. 79 C, D, 



268 PHYSIOGRAPHY. [chap. 

The exact nature of these very curious bodies is not at 
present known. 

In addition to the calcareous organic remains which 
constitute the greater part of the ooze, it contains multi- 
tudes of siliceous skeletons, some of which belong to simple 
animal forms, such as Radiolaria and sponges, while others 
are vegetable organisms, belonging to the group of Diatoms 
described in the last chapter. The Diatoms and the Radi- 
olaria inhabit the surface of the ocean, along with the 
GlobigerincB and Orbiiiince, but the sponges live at the 
bottom. Here and there, the remains of other animals 
which inhabit the depths of the sea, such as starfishes, 
sea-urchins, and various sliell-fish, are also imbedded in 
the ooze, and contribute to the sohd submarine deposit. 

It is very interesting to remark that, just as the process of 
pluvial denudation is only, in part, a conversion of solid into 
fluid matter, and, for the rest, effects a mere transference of 
solids ; so, the process of reconstruction of solids, which 
takes place in the superficial parts of the ocean, by the 
agency of the GlobigerincB, is not permanent. In other 
words, there is reason to believe that the Globigerijia shells 
in the ooze, at the bottom of the sea, do not represent all 
the work in the way of withdrawing calcareous matter 
from solution in sea-water, which has been done by the 
Globigerince at its surface. 

It has been seen that living Globigei-i?i(2 are found in 
the uppermost stratum of the sea, all over the warm and 
temperate parts of the world. Hence, it would seem to 
follow, that Globigerina-ooze should be found covering the 
bottom of the sea over the whole of these regions \ and, in 
fact, it is met with, at all depths, between 250 and 2,900 
fathoms, over an immense extent of both the Atlantic and 
the Pacific oceans. 



XVI.] FORAMINIFERAL LAND. 269 

But there are some areas of these oceans, occupying many 
thousand square miles, in which the sea-bottom is covered, 
not with Globigerina-ooze, but with a red mud, which 
appears to be nothing but clay in a very finely divided 
state. These areas are usually met with only at a very 
great depth, over 2,500 fathoms in fact; and the naturalists 
of the Challenger observed that, in passing from the adjacent 
region covered with the ordinary Globigerina-ooze, into one 
of these red-clay areas, a region, covered with a sort of grey 
mud (" grey ooze "), intermediate in its character between 
the Globigerina-ooze and the red clay, was traversed. Where 
the grey ooze began, the Globigerina shells appeared to be 
corroded, as if they had been attacked by an acid ; and, as 
the red clay was approached, they became more and more 
fragmentary, and at length altogether disappeared. 

There can be no doubt that the foraminiferal shower 
falls over the area, occupied by the grey ooze and the red 
clay, just as persistently as elsewhere. What then becomes 
of the shells ? There seems to be no escape from the 
conclusion that the calcareous matter of which they are 
composed is dissolved away. The Globigerince are so 
minute, that their skeletons must take a great length of time 
to subside through the three or four miles of water, which 
overlie the deeper sea-bottoms. But sea-water contains 
much carbonic acid ; and, it has already been seen, that 
carbonate of lime, especially if finely divided, is soluble in 
such water. Hence, it is highly probable, that the fora- 
miniferal shower is, in part, redissolved before it reaches the 
bottom ; and that, other things being equal, the greater the 
depth, the greater will be the loss suffered in this manner. 

The difficulty is to understand, not why the GlobigerincB 
should disappear from the bottom of the very deep parts of 
the ocean, but why the process of solution should be so 



270 PHYSIOGRAPHY. [chap. 

much hastened, at depths between 2,500 and 3,000 fathoms, 
that an abundant residuum of undissolved shells is left at 
the former depth, and none at all at the latter. Here is a 
question which cannot as yet be answered. 

Again, what is the "red clay" which takes the place of 
the Globigerina ooze ? It has been suggested that it is the 
residuum left after the Globigerince have been dissolved; 
but there is no sufficient evidence that pure and clean 
GlobigeiHna shells contain any appreciable proportion of 
such mineral matter. 

An alternative supposition is, that the red clay is simply 
the finest of the washings of the land, which have gradually 
drifted into the greatest depths of the ocean ; while another 
explanation which has been offered is, that it is the result of 
the decomposition of the volcanic ejections which are borne 
about by the winds, and finally scattered over the surface of 
the ocean ; and which, as a matter of fact, are found floating, 
far and wide, in the shape of pumice. Fragments of 
volcanic minerals are everywhere found in the Globigerina 
ooze ; and it is highly probable that the products of 
"volcanic showers" are intermixed with the foraminiferal 
shower all over the ocean. If this be the case, then, in 
those localities in which the Forami nifet'a are dissolved 
before they reach the bottom the volcanic minerals would 
remain as the sole constituent of the ooze ; and, by their 
decomposition, they might give rise to the red clay. 

From what has been said it follows that, if, in consequence 
of one of these movements of upheaval to which reference 
has been made, the present bed of the Atlantic were raised 
to the surface and became dry land, the many thousand 
square miles of new dry land thus produced would be 
found to be covered, for an unknown thickness (amounting 
possibly, and indeed probably, to hundreds of feet), with a 



XVI. J FORAMINIFERAL LAND. ■ 271 

bed ot softish limestone. The great bulk of this calcareous 
rock would be made up of entire, or fragmentary, Globi- 
gerina and Orbulina shells ; but it would contain, in addi- 
tion, other Foraniinifera, shells of shellfish, remains of 
starfishes and sea-urchins, and of such other animals pro- 
vided with hard skeletons as are now living in the depths 
of the Atlantic. 

It would, in fact, be a highly " fossiliferous " limestone 
with more or less silex, in the shape of Radiolaria 
skeletons and sponge spicules, scattered through its mass, 
and would constitute an element of no small magnitude 
and importance in the compostdon of the earth's crust. 



CHAPTER XVII. 

THE GEOLOGICAL STRUCTURE OF THE BASIN OF THE THAMES ; 
AND THE INTERPRETATION OF THAT STRUJTURE. 

In the preceding chapters, the general character of the 
River Thames and the form of the surface which it drains, 
have been considered ; its waters have been followed to the 
sea, and, thence, by way of the atmosphere, back to that 
surface ; while the atmosphere, and the waters of the land 
and sea, have been traced back to the elementary bodies of 
which they are composed. The river, and the rains which 
feed it, were next considered as a giinding and dissolving 
machinery, by which the surface of the Thames basin is 
being insensibly worn away and its materials carried down 
to the ocean ; while the sea, so far as it washes the banks 
and shallows of the estuary and of the adjacent coasts, was 
shown to be a no less persistent destroyer of the dry land. 
And then, seeing that all rivers and all oceans are engaged 
in the business of denudation and dissolution, it became a 
matter of interest to discover what natural operations, if 
any, tend to compensate this constant wear and tear of the 
dry land. Such compensating agents were found in the 
forces which tend to raise submerged land ; in volcanoes, 
which transfer fluid matter to the surface, where they 



CH. XVII.] GEOLOGY OF THE THAMES BASIN. 273 

solidify ; and lastly in living matter ; which, on the whole, 
tends constantly to increase the solids of the globe, at the 
expense of its fluid and gaseous components. 

With these conceptions of the general nature of the 
agents which are now at work in modifying the crust of the 
earth, it will be possible to start with profit upon a new 
series of considerations. 

The Thames basin presents, as has been seen, a surface 
diversified with hills and valleys ; this surface is everywhere 
covered with a comparatively thin layer of soil, which, in 
many places has been more or less alteied in character by the 
builder, the road-maker, or the farmer, and is then known as 
"made ground." But beneath this lies the subsoil, which forms 
the uppermost part of the solid floor of the basin. It has been 
Been that this subsoil varies very much at different places, being 
here gravel or sand, there clay, in another place chalk, in 
another a different kind of calcareous rock. Moreover, it has 
been incidentally mentioned, that these materials are arranged 
in layers or strata ; so that, if the floor of the Thames basin 
could be cut down vertically, the faces of the section would 
present a succession of layers, one above the other. It has 
been mentioned that quarries and railway cuttings, here and 
there, afford the opportunity of examining the strata in their 
natural relations and order of superposition. Sections of 
this kind afford direct evidence of the structure of the earth 
for only a very little way below the surface ; but more is to 
be learned from the deep borings for wells, artesian and 
other, which have been referred to in Chapter II. 

Such borings have been carried to a depth of 1,300 feet,-^ 
and they show that the subsoil of the Thames basin, in and 

^ A famous boring for water at Kentish Towti, in 1854, was ciinied 
down to a depth of 1,302 feet. The boring passed through 236 feet of 
London clay ; 88J feet of Lower London Tertiaries ; 645 feet of chalk ; 

T 



274 PHYSIOGRAPHY. [chap. 

about London, is everywhere made up of beds of gravel, 
sand, and clay of varying thickness ; these rest upon a thick 
bed of chalk ; and, beneath this, follow strata of sand- 
stone, hardened clay, and calcareous rocks of a totally 
different character from the chalk. Whether we travel to 
the north, the south, and the west, or the east of London, 
we find the bed of chalk, which underlies London at a depth 
of upwards of 300 feet, coming up to the surface. In other 
words, the layer of chalk, beneath the Thames basin, is bent 
up, on all sides, in such a manner as to have the form of a 
very shallow dish,^ the bottom of which is covered with 
horizontal layers of sand and clay, while its eastern end 
is notched by the estuary of the Thames. Passing over 
the upturned edges of the layer of chalk on the north, the 
west, and the south, other rocks, as we have seen, lie at 
the surface; and, some of these, such as the greensand 
end the gault, are of the same nature as those which 
follow on the chalk in the vertical borings. It is obvious, 
therefore, that the stratum of chalk lies on the greensand 
and gault strata, just as a basin fits inside one a size larger. 
In the western part of the Thames basin, it has been seen 
that the subsoil rocks consist of limestones, sandstones, and 
clays. These beds are found beneath the chalk, greensand, 
and gault, some distance to the eastward. Underneath 
London, however, they are absent ; for the borings which 
have been carried deep enough to traverse the chalk, green- 
sand, and gault, enter rocks which are unlike any found at 

13 J feet of upper greensand; 130J feet of gault; and 1884 feet of clays, 
sandstones, and conglomerates, which were of doubtful age. 

1 The form into which the layer of chalk is bent is altogether inde- 
pendent of that of the Thames basin itself, although the two happen, 
to a certain extent, to correspond. Every area drained by a river has a 
more or less dish-like form, whatever may be the arrangement of the 
strata which ccnstitute its floor. 



XVII.] GEOLOGY OF THE THAMES BASIN. 



275 



the surface of the Thames basin ; though strata of a similar 
character appear at the surface, further to the west. All the 
different strata which thus make up the floor of the Thames 
basin contain fossil remains of animals or of plants, or of 
both, sometimes in great abundance. 

Such is a broad and general statement of the facts which 
have been ascertained respecting the structure of the floor of 
the basin of the Thames. How are they to be interpreted ? 
Some light may be thrown upon this question by considering 
the method which is pursued by antiquaries and archgeO' 
logists, in order to extract trustworthy history from the works 
of men. 



A-^ 










Fig. 80.— Section exposed in Cannon Street, London, 1851. 



In 1851, during some improvements which were being 
made in Cannon Street, in the city of London, a deep 
digging exposed a section such as is represented in Fig. 80.1 
At some distance below the level of the street, A, an old 
pavement, B, was found; while, deeper than this, and 
separated from it by a considerable quantity of soil, C, 

^ This section was recorded at the time by Mr. Chaffers. See his 
Marks and Monograms on Pottery. 

T 2 



276 PHYSIOGRAPHY. [chap. 

was another pavement, D, of the kind which is termed 
tessellated ; and underlying this was another bed of soil, E. 

There can be no reasonable doubt that the modern road- 
way, A, was laid down after the old pavement, B, and 
that this pavement again is of later date than the tessellated 
pavement. The three layers of soil, those between the 
pavements and that above the pavement, B, contained 
fragments of pottery, coins, and other articles, such as are 
apt to accumulate in the rubbish of great cities. 

Taking all the circumstances of the case into considera- 
tion, this Cannon-Sti-eet section affords to the archaeologist 
sufficient grounds for the conclusion, that human beings 
occupied this locality for a very long period ; though it 
would be quite impossible to say how long, without inde- 
pendent evidence. Moreover, as the pottery and other 
relics in the layer E, are of a totally different character 
from those in the layer C, the archaeologist would be 
justified in supposing, either that the people who inhabited 
the locahty during the time represented by E, were of 
different race from those who inhabited it during the time 
represented by C ; or, if the same, that their posterity had 
undergone some great change. 

If the relics in the layers E and C, were unlike any- 
thing known, the conclusions of the archaeologist thus far 
would be quite justifiable, but he could get very little further. 
As a matter of fact, however, when the relics in the bed E 
are compared with what are known, on independent grounds, 
to be the coinage and pottery-work of the ancient Romans, 
they can be identified with the latter ; while the tessellated 
pavement is no less characteristically Roman. On the other 
hand, the coins and other objects in the stratum C, have all 
the characters of those which are known, from independent 
evidence, to have been produced in England, in the period 



XVII.] GEOLOGY OF THE THAjMES BASIN. 277 

which ranges from the Norman conquest to the sixteenth 
century. Hence it might be concluded that the pavement 
B, was put down, at least as late as the sixteenth century ; 
and this inference is confirmed by the fact, that such a 
pavement was known to exist in the locality, before the 
great fire of 1666. 

Thus it will be observed, that the general conclusions de- 
duced from the character and contents of the successive beds 
of made ground in Cannon Street, are fuhy borne out by 
independent evidence ; and, if nothing were known of the 
ancient history of England, beyond such archaeological facts 
as these, they would leave no doubt as to the fact, and the 
relative age, of the Roman occupation. 

The principles, on which the interpretation of the Cannon 
Street section rests, are two : ist. In such cases as that under 
consideration, the uppermost stratum is the latest formed, 
and the lowest, the oldest. 2nd. The similarity of bodies 
having a definite form and structure is presumptive evidence 
of the similarity of their origin. It is the former principle 
which justifies the conclusion that the pavement D is older 
than the pavement B ; it is the latter, which leads us to 
say that a piece of pottery is Roman rather than English. 
All the conclusions as to the history of the earth, deduced 
from the structure of its crust, are based upon analogous 
principles. If certain strata can be shown to have been 
deposited by aqueous agencies, then the uppermost of these 
strata' is the newest, and the undermost is the oldest. It 
the fossils which are em.bedded in these strata can be 
proved to be similar, in all essential respects, to the hard 
parts of living animals and plants, then they are evidence 

■ In much disturbed regions of the earth's surface the strata may be 
locally termed upside down, so that the newest are underneath ; but 
this does not affect the general principle. 



278 PHYSIOGRAPHY. [cH. xvii. 

of the existence of such animals and plants, antecedent to, 
or during the deposition of, these strata. 

On referring again to the Cannon Street section, it is seen 
that below those deposits of soil, which show, by the charac- 
ter of their embedded remains, that they are of Roman date, 
there is first a thin layer of clay, and then a deposit of gravel, 
F, These lower beds did not yield, at this locality, any 
relics of human workmanship ; and, indeed, the explorer 
who examines them soon finds that he is dealing with 
deposits in which neither Englishman, Roman, nor Briton 
has left any mark. 

The gravel exposed in this section forms part of a wide- 
spread sheet, which covers a large portion of the valley of 
the lower Thames. Its range in the neighbourhood of the 
metropolis is shown on Fig. 8i ; which is a map, giving the 
area of the superficial deposits, or "drifts," as they are 
sometimes called, ^ in the Thames valley between Kingston 
and Woolwich, The dotted part shows the gravel as 
exposed at the surface. This gravel consists, principally, of 
rounded and subangular pieces of flint, derived from the 
chalk, the spaces between the stones being mostly filled with 
sand. The origin of so vast a deposit of gravel is by no 
means clear ; but there is reason to believe that much of it 
is an old river-gravel, formed by the Thames when the river 
flowed at a greater height, and, probably, with much larger 
volume, than at the present day. 

Along the banks of the Thames, and of most other rivers, 
it is not uncommon to find successive terraces of gravel, 
which mark the height at which the river flowed at different 
periods. Thus in Fig. 82 (which is a section from Wimbledon 

' So called because it was formerly believed that such deposits had 
been driven over the surface by great floods. The word is conveni 
ently retained without any reference to the origin of such deposits. 



28o PHYSIOGRAPHY. [chap. 

to Wandsworth Common across the river Wandle, similar 
to that given on p. 139) it is believed that the river (R) 
ran at one period over the higher terrace, No. i, and 
then cut its way down to the lower, No. 2. The higher 
deposit is consequently the older, and is distinguished some- 
times as the high-level g7-avel ; while the lower deposit, which 
is of more recent age, is known as the low-level or valley 
gravel. In a section of strata, such as that in Cannon 
Street, the uppermost beds are the most recent; but, on 
the banks of a river, the higher deposits are presumably 
the oldest, and will contain the remains of animals that 
inhabited the valley before the river reached its lower level.^ 





Fig. 82.— Section across the Valley of the Wandie, showing high-level gravels and 
valley-gravel. 

In many places around London, the sheet of gravel is 
overlaid by a thin deposit of brownish loam^ represented on 
the map as hrick-ea7'th^ since it is largely worked by brick- 
makers. This earth forms an excellent soil for vegetables, 
and many of the market-gardens at Fulham and elsewhere 
are situated upon it. It is probable that this brick-earth 
has been thrown down by the river in flood. When the 
Thames has overflowed its banks, it has deposited silt upon 
the neighbouring land ; or, possibly, there may have been a 

^ There is no real contradiction here. The higher gravels of the 
river valley do not lie upon, but only at a higher level than, the lower 
gravels. There are no exceptions to the rule, that of two strata, one 
snperimpoFed on the other, deposited by water action, the upper is the 
more recent, if the beds have not been disturbed since their deposition. 

* Loam is a sandy clay ; marl a calcareous clay. 



^ivii.] GEOLOGY OF THE THAMES BASIN. 281 

lime when the river spread out, at certain parts of its course, 
into wide lake-like areas, and quietly deposited mud and 
sand at the bottom of these sheets of water. The strips of 
alluvium (p. 142) deposited by the river along its margins, in 
comparatively modern times, are also indicated on the map. 
Fig. 81. This marshy land, bordering the Thames and it? 
tributaries, spreads out below London into wide flats ; and 
it occasionally contains shells, bones and other organic 
remains. Of the vegetable relics, which are found in some 
of the marshes, and indicate the site of an ancient forest, no 
mention need be made here, since they have already been 
noticed at p. 212. 

Fossils are not confined, however, to the comparatively 





Fig. 83. — Cyrena [Corhicula) JhciniiiaUs. 

modern mud which forms the alluvium of the Thames ; but 
they are also found, more or less abundantly, in the older 
superficial deposits, such as the gravels and brick-earths. 
Thus, they are especially abundant in the brick-earth which 
is worked at Frith and Crayford in Kent, and at Ilford and 
Grays in Essex; while, above London, they have been found 
in the sands and loam of Brentford. Many of these fossils 
are land and fresh-water shells, which once lived in the 
river and on its banks ; and are, for the most part, not 
different from those living in the locality at the present day. 
Some few of the shell-fish, however, have long ceased to 
dwell in the rivers of this country, though still found in 



282 PHYSIOGRAPHY. [chap. 

other parts of the world. Such, for example, is the little 
shell represented in Fig. 83, and known as Cyrena, or 
Corbicula, flumhialis. This is not uncommon in the old 
deposits of the Thames, but is not living at the present time 
in any English, or indeed in any European river, though 
it is still found in the Nile and in Kashmir. 

While the shells of the old Thames deposits represent 
species, most of which are still living in Britain, it is far 
otherwise with the bones which are found in the same beds. 
Many of these bones, indeed, are those of animals extremely 
different from any which now inhabit this country, or are 
known to have inhabited it within historical times. And, 
yet, there can be no doubt that the animals which have left 
these remains, once lived and died within the area of the 
Thames basin. Just as the coins or the pieces of pottery 
which are found in old " made ground " beneath London, 
are unquestionable relics of the people who dwelt in the 
city, when the soil was in course of accumulation ; so, these 
bones represent the animals which roamed over the Thames 
valley during the period when the deposits in which they 
occur were in course of formation. 

When the brick-earths of Kent and Essex were being 
deposited, the fauna, or animal population, of the Thames 
basin, included, in addition to many animals still living 
here, a number of extinct mammals,^ such as the mammoth 
{Ekp/ias primigenius) ; this was a kind of elephant adapted 
to live in a cold climate by having a thick woolly coat. 
Fig. 84 represents a restoration of this extinct elephant."^ 
Another species of elephant (£. antiquus) also lived in the 

■^ Manwials, from Lat. vianima, breast ; a great group of back-boned 
animals which suckle their young. 

^ This is reduced from Brandt's figure in his Mittheilungen iiber die 
N'aturgeschichte des Ma7nmuth oder Mamont. St. Petersburg. 1866. 



XVII.] GEOLOGY OF THE THAMES BASIN. 283 

Thames valley ; and along with the elephants were three 
distinct kinds of rhinoceros (i?. tichurhhius, R. niegarhmus, 
and R. hemitcecJms). All these animals are extinct ; but the 
hippopotamus which lived in the ancient Thames is not to 
be distinguished from that now dwelling in Africa. The 
brick-earths also contain the remains of a species of lion 
{Felis spelcea), no longer living, but which is likewise found 
in some of the bone-caves of this country. Among other 
animals which lived here, at the same period, may be 




Fig. 84. — The hlavaxnolh {Elejihas priiHig;enius). 



mentioned the brown bear, the grizzly bear, the spotted 
hysena, and two kinds of large wild oxen, — the bison and 
the urus. The gigantic Irish " elk " {Cervus megaceros), which 
is now extinct, has also left its bones in the brick-earth ; and 
Professor Boyd Dawkins found, at Crayford, a skull of the 
musk-sheep [Qvibos moschatus), which is a creature living at 
the present day only in Arctic America. Most of these 
are represented, not by a mere bone or two, indicating an 
occasional straggler, but by remains so abundant as to show 
that the animals which they represent were important 



284 PHYSIOGRAPHY. [chap. 

members of the old fauna. Thus, the collection of Sir 
Antonio Brady contains portions of no fev^er than a hundred 
elephants, all collected from the brick-earth of Ilford. 

Such, then, is the curious assemblage of animals which 
fed at one time in the valley of the Thames, and have left 
their bones and teeth in the ancient deposits of the river. 
Some of these mammals have since died out, and are no 
longer to be found in any part of the world ; others have 
wandered to the south ; while others, again, have retreated 
northwards, a few, however, still remaining in the present 
fauna of the country. The strange association, in the same 
deposit, of both northern and southern forms — these in- 
dicating a warmer, and those a colder climate — offers per- 
plexing evidence as to the climatic condition of the country 
at the time in which- they lived. It is certain, however, that 
at one time, the climate of the Thames basin must have 
been extremely severe, since some of the deposits in the 
northern districts present unmistakable evidence of the 
prevalence of glacial conditions (p. 165). Evidence of the 
kind given in Chapter X. shows that there must have been a 
time when Britain, north of the Thames, was covered either 
with land-ice, or with an icy sea, from which the boulder clay 
and glacial gravels were deposited. Possibly, some of the 
gravel in the Thames basin may be glacial drift, which has 
been disturbed and re-deposited by the river. And, it should 
be mentioned, that the remains of the reindeer are abundant 
in many of the superficial deposits of the Thames valley, 
though not in the brick-earth, which has yielded so many of 
the fossils previously noticed. Indeed, the relation of this 
brick-earth to the glacial period is by no means clear ; some 
geologists believing it to be more, and others less, recent 
than the true glacial drifts. 

It becomes an extremely interesting question to determine 



XVII.] GEOLOGY OF THE THAMES BASIN. 2S5 

whether man shared possession of the Thames valley with 
the group of animals, the remains of which are found in the 
old fluviatile deposits of gravel and brick-earth. In the British 
Museum, there has been, for many years, a rude spear-shaped 
weapon in black flint, represented in Fig. 85. This was 
found, associated with an elephant's tooth, in an excavation 
near Gray's Inn Lane, London ; and a description of it was 
published as far back as 1715. It is indeed the earliest 
recorded relic of human workmanship which has been found 




-A palaeolithic implement, I'iG. 86. — A neoliihic implement 

from Gray's Inn Lane. from the Thames at London. 



in association with the ancient fauna of the Thames Valley. 
Of late years, however, considerable attention has been 
given to the subject, and many other flint implements have 
been discovered in the high-level gravels of the Thames 
basin. Acton, Ealing, Hackney and Highbury are localities 
near London which have yielded such implements ; and 
they have also been found in numbers, between Heme Bay 
and the Reculvers, where they have fallen out of the gravel 
which caps the chalk cliffs of the North-Kent coast. Even 



286 PHYSIOGRAPHY. [chap. 

the brick-earths of Crayford and Erith have yielded a worked 
flint or two ; though no implements of the type noticed 
above. So many of these flints have been found in the old 
gravels, not only in the Thames Valley but in various other 
parts of England, and also in the valleys of northern 
France, that there is no reason to doubt the existence of 
man, in this part of the world, during the period at which 
the older river-drifts were deposited. Moreover, some of 
these implements have been found in such close associa- 
tion with the bones of extinct animals, that there is equally 
little doubt as to the co-existence of man with the old fauna 
of this period. It is probable, indeed, that the early flint- 
using man came hither from the continent with some of the 
extinct mammalia ; at a time when Britain was connected 
with the European mainland by an isthmus occupying the 
position of what is now the Straits of Dover. 

Flint implements, such as that represented in Fig. 85, 
art the oldest known relics of man. They indicate a time, 
before the commencement of history in Western Europe, 
when man was ignorant of the use of metal, and fashioned 
his weapons and implements out of stone. The more 
ancient of these prehistoric implements, such as that in 
Fig. 85, are simply chipped into shape; but other stone 
implements occur, which are neatly ground, and even 
polished. Fig. 86 represents a stone celt'^ which was 
dredged up from the Thames, at London, and is now in 
Mr. Evans's collection. ^ These more highly finished stone 
implements are never found in the old high-level gravels, 

^ Celts, from Lat. celtis, a chisel ; not, as often supposed, because 
they were used by the people called Celts. 

^ Figs. 85 and 86 are reduced from figures in Tke Ancient Stotte 
Implements, Weapons, and Ornaments of Great Briiain. By John 
Evans, F.R.S., &c. 1872. 



XVII.] GEOLOGY OF THE THAMES BASIN. 287 

or along with the extinct mammalia ; but are confined to the 
most superficial deposits, and to the present level of the 
river. That period in which man was in the habit of using 
implements exclusively of stone is known to antiquaries as 
the stone age ; and Sir John Lubbock, has distinguished the 
early part of this period, in which unpolished stone was in 
use, as \\\t palcEoUthic'^ age, and the later period, when man 
had advanced to the stage of grinding and polishing his 
weapons, as the Jieolithic'^ age. Fig. 85 represents therefore 
a paleolithic, and Fig. 86 a neolithic implement. 

All the deposits hitherto described in this chapter — such 
as the gravels and brick-earths — consist of loose materials 
distributed in patches, more or less extensive, over the 
surface of the solid rock. Hence, they are classed together 
by the geologist as " superficial deposits ; " and are not 
represented on a geological map, unless the map be con- 
structed for the special purpose of exhibiting the surface- 
geology, as in the case of the little sketch-map, Fig. 81. 
An ordinary geological map shows, in fact, the kind of rock 
which would be exposed on the surface of the ground, if the 
superficial deposits were removed. In some cases, there 
are no deposits of this kind, and then, of course, the actual 
rock of the country is exposed. The map of the basin of 
the Thames given in Plate V., is coloured, in such a 
manner, as to show what rocks would be seen on the face 
of the country, if it were not obscured by gravels, loams, 
and other superficial accumulations. 

The small areas of light brown tint in Surrey and Berks 
represent the highest, and therefore the newest, beds within 
the Thames basin. They are well seen in the sandy tracts 
of Bagshot and Ascot Heaths ; and, from the former of these 

^ Palccolithic, from iraAaios, palaios, old ; \ldos, lithoSy stone. 
* Neolithic, from j/e'os, neos, new. 



288 PHYSIOGRAPHY. [chap. 

localities, the deposits in question have received their name 
of Bags/tot beds. It has been already explained (p. 25) 
that the hills of Hampstead, Highgate, and Harrow are 
capped by Bagshot sand ; but these areas are too small to 
be represented on the map. Very few fossils occur in these 
sands, but those which have occasionally been found are 
fragments of marine shells, thus showing that the area m 
which they occur has been, at some time, under the sea. 
There is little doubt that the Bagshot sands were once spread 
over a wide surface in the lower part of the Thames basin, 
and that a great portion of them has since been removed by 
deundation. 

Reference to Fig. 7 (p. 26) will show how the Bagshot 
sand usually rests upon the London Clay. This clay, which 
is the next rock in passing downwards from the Bagshot 
beds, is represented in Plate V. by a dark-brown colour, and 
is seen to cover a very wide area. It is, for the most part, 
a stiff brown clay, which has evidently been deposited, as 
fine mud, upon the bottom of the sea, not far from land. 
In fact, in the Isle of Sheppey, the clay has yielded a great 
variety of vegetable remains, some of which indicate a very 
warm, not to say tropical, climate. Thus the fruit of the 
Nipadites represented in Fig. di (p. 230) has its modern 
representatives in Bengal and the Asiatic Archipelago. 
Such relics of terrestrial vegetation indicate that land could 
not have been far from the water in which the clay was de- 
posited; and the occurrence of the bones of crocodiles also 
tends to the conclusion that the Sheppey clay represents the 
delta of some ancient river. On passing from Sheppey 
towards London, and farther to the west, the vegetable fossils 
of the London clay disappear, while marine shells are to be 
found locally, as at Highgate. I^Iany of these shells, though 
they belong to extinct animals, resemble those which are 



Plate V; 



1^0 rtt DcrwiLS 




OF COiOURS 



li! 


11 


BAGSHOT 

Beds 


r 1 


London clay 

ANO LOWER 
LOmiOnTERTIARItS 








Chalk 








Grcensand 

(UFPCR & LOWEH) 
AND GAULT 






i - 


WEALD Clay 




Hastings 

SAND 


liiifef 






Oolite 



Li A3 



Tri AS 




3ealeveL 



Basin . 



Y ^ 




tV ^ 






Fold-out 
Placeholde 



This fold-out is being digitized, and will be 

future date. 




VII.] GEOLOGY OF THE THAMES BASIN. 2S9 

confined at the present day to warmer seas; Fig. 87, for 
example, is a fossil Nautilus, a genus which is represented 
by several species in the London clay. When it is re- 
membered that, beneath London, this clay is about 400 
feet thick, it will be readily admitted that such a deposit 
of fine muddy matter must have required an enormous 
period of time for its deposition. Moreover, it must be 
borne in mind, that, in the case of this clay, as of so many 
other sedimentary deposits, the present thickness does not 
necessarily indicate its original thick- 
ness, for much may have been re- 
moved by denudation. 

Beneath the London clay of the 
Thames basin, there is a series of 
comparatively thin beds, known as 
the Loiver London Tertiaries. These ^ 

come to the surface along the edge -^^-^ 

c .^ ■[ v • r ii Fig. 87. — Naiitilus centralis, 

of the clay, as it rises from the from the London day. 

margin of the basin, but their 

width is too small to admit of representation on the map 
in Plate V. Their position, however, is indicated in Fig. 1 1 , 
(p. 31). The uppermost members of the group consist of 
rolled flint pebbles, which evidently represent an old beacli 
of shingle, associated with sands, which often contain marine 
shells. These beds are known, from the localities in which 
they are best exposed, as the Blackheath or Oldhaveji beds. 
They are succeeded, below, by clays, some of which are rich 
in shells similar to those now living in brackish water; whence 
it is concluded, that the beds must have been deposited in 
an estuary. From the two localities in which these strata 
are typically developed, they are known as the Woolwich 
and Reading beds. Below these, come the T/ianet beds, of 
which good exposures may be seen between Heme Bay and 

u 



2SO PHYSIOGRAPHY. [chap. 

the Reculvers, and in Pegweli Bay, in the Isle of Thanet, 
where they consist of sands containhig marine shells. 

All the strata noticed above are classed together in one 
great group known as the Tertiaty or Cainozoic ^ series. The 
former name refers to the fact that geologists have been led, 
m their study of the earth's crust, to recognize three great 
groups of rocks, of which, those hitherto described in thirj 
chapter, represent the uppermost or third, reckoning from 
below upwards. The Tertiary series, as developed in the 
London basin, comprises all rocks, from the Thanet sands 
below, to the Bagshot beds above. Over this Tertiary 
group, come the glacial-drifts, and river- gravels, which are 
sometimes grouped together as a fourth set of deposits, a id 
are consequently known as the Qnafernajy series, while by 
others they are called Post-tertiary formations. While the 
Tertiary beds are succeeded above by the quaternary series, 
they are followed below by another great group, known as 
the Secondary or Alesozoic'^ series, of which the uppermost 
member is the well-known chalk. 

The pale-green tint on the map (Plate V.) covers a large 
area occupied superficially by the Chalk; while the rela- 
tion of this rock to the overlying Tertiaries may be seen 
in the coloured section in the same plate,^ and has already 
been shown diagrammatically in Fig. ii (p. 31). The 
ground formed by the chalk usually consists, when not 
covered with drift, of gently undulating downs, carpeted 
with soft turf. Good examples of chalk scenery may be 
seen in the North Downs of Surrey and Kent, and in 

^ Cainozoic, from Kawos, kainos, recent ; C^ov, zoon, an orgamsni. 

^ Mesozoic, from fxia-os, mesos, middle, 

3 This section, drawn by Mr, Wliitaker, of the Geological Survey, is 
supposed to b^ taken along the line, A B, on the map. The vertical 
scale of the section is about twelve times gi-eater than the horizontal 
cale. and the latter is evidently larger than that of the map. 



XVII.] GEOLOGY OF THE THAMES BASIN. 291 

Salisbury Plain ; while, along the south-eastern coast, it 
forms those dazzling white cliffs which gained for this island 
the old name of Albion. The chalk, which, beneath London, 
is 600 or 700 feet thick, is chiefly composed of carbonate of 
lime, in some parts interspersed with beds of flints, which 
are nearly pure silex. There cannot be the least doubt 
that the chalk represents the mud of an ancient sea-bottom, 
for multitudes of remains of animals have been obtained 
from it, most of which belong to such groups as are ex- 
clusively marine at the present day. The area once occu- 
pied by the chalk was therefore, at one time, covered 
by sea. And it may be further concluded that it was at 
some distance from any extensive land, inasmuch as the 
chalk contains no such mixture of clay and sand, as would 
be derived from denudation. But there is another reason 
for believing that the chalk ocean was pretty deep ; that 
is to say, over 100 fathoms. If a slice of chalk is cut, 
ground thin, and mounted in Canada balsam, so as tc/ 
become transparent, shells of Foraminifera may almost 
always be detected in it; and, sometimes, they abound. The 
commonest form is a Globigerina, indistinguishable from 
that which constitutes the bulk of the Atlantic ooze. 
Moreover, coccoliths and coccospheres are plentiful in the 
chalk : which thus differs from the ooze, chiefly, in the 
greater proportion of granular particles, without any de- 
finite shape, to recognizable organic remains ; and, in the 
entire absence of those siliceous shells and skeletons, which 
are so constant in the ooze (Figs. 88 and 89).^ 

The former difference presents no difficulty. The chalk 

^ Fig. 88 represents a section of chalk from Little Hampton, on the 
coast of Sussex, while Fig. 89, which is placed by its side for compari- 
son, represents a sample of Atlan<;ic ooze, taken by Captain Dayman 
from a depth of 2,250 fathoms. Both figures are magnified to the same 
extent. 



292 



PHYSIOGRAPHY. 



[chap. 



may have been formed in just the same way as the Globi- 
gerina ooze, which is now being deposited ; and the pro- 
portion of shells which have been broken down into mere 
dust, may have been increased by the pressure to which it 
has been subjected ; while, in some parts, the original 
structure may have been often more or less completely 
obliterated by the percolation of water; just as in coral- 
rock, the shapes of the component corals become lost, 
and in diatomaceous deposits (Chap. XIV.) the individual 
diatoms run into a sort of opal, from the same cause. 




Fig. Si. —Microscopic section of cnalk 
from Sussex. Magnified about 
220 diameters. 



J. 89. — Atlantic ooze from a depth 
of 2,250 fathoms. Magnified 
about 220 diameters. 



With regard to the second difference, it may be remarked, 
that there is no reason for doubting that the ocean, under 
which the chalk was deposited, contained as great an abun- 
dance of organisms with siliceous coats and skeletons, as 
the present Atlantic. The conclusion is, therefore, that 
such siliceous remains have once existed in the chalk, but 
have been dissolved ; and it is supported by the fact, that, 
even the sponges, the remains of which are found in the 
chalk ill great abundance, have lost the siliceous spicula 



XVII.] GEOLOGY OF THE THAMES BASIN. 293 

which existing sponges of similar kinds always contain. 
On the other hand, the chalk contains flints, of which no 
trace is found in the ooze. On the whole, it is probable, 
that these flints represent the siliceous organisms which 
were contained in the Cretaceous ooze, when it was de 
posited, but which have been dissolved up by percolating 
water, and re-deposited in the shape of amorphous silex ; 
just as the diatoms in the Bilin beds have been dissolved 
and re-deposited as opal. 

The lowermost beds of the chalk rest upon sandy de- 
posits, which are termed the Upper Greensand. Some of 
these sandy beds are not the mere mechanical detritus of 
sihceous rocks, but contain numerous greenish grains of 
definite and well-marked forms. They are, in fact, the casts 
of the internal cavities of foraminiferal shells in a compound 
of silica, iron, and clay, which is known as silicate of iron 
and aluminium. This substance has been deposited in the 
cavities of the shells, the calcareous matter of which has 
been subsequently dissolved away, and has left the hard 
cast behind. A similar sandy deposit is taking place ofi^ 
the eastern coast of the United States, upon the Agulhas 
bank, near the Cape of Good Hope, and elsewhere, at the 
present time, in depths varying from 100 to 700 fathoms. 
A tolerably deep sea, therefore, covered the area now 
occupied by the chalk, not only during the time which 
was occupied by the deposition of the chalk, but ante- 
cedently to that period. 

But we can carry the evidence of the existence of a 
sea covering the western part of the Thames basin, to a 
still more remote period. The organic remains which are 
found in the lower cretaceous strata and in the oolitic and 
liassic beds, which underlie the chalk, and are exposed in 
this region, are chiefly those of marine animals. In thr; 



294 PHYSIOGRAPHY. [chap 

neighbourhood of Oxford, beds of the oohtic series are 
exposed, which are so rich in fossil corals that they go by 
the name of the Coralrag. These corals resemble those 
which are now forming reefs, and the coral-rag itself is 
altogether similar to modern coral-limestone ; so that there 
can be no reasonable doubt tliat the coral-rag is the product 
of the reefs of a sea, which covered this region long before 
the chalk began to be deposited. 

Putting all these facts together, it becomes clear that 
the present condition of the basin of the Thames was 
preceded by one in which the river flowed at a higher level 
.and the climate was much more extreme, if not much colder, 
than it is at present, during which the quaternary deposits 
were formed. Antecedent to this, was a period in which 
the region, at present covered by the London clay, was a 
great estuary, and the climate was much warmer than at 
present. This was preceded by the period during which the 
chalk was deposited, when the greater part, if not the whole, 
of the Thames basin was far beneath the surface of the sea ; 
and a similar condition appears to have obtained as far back 
as positive evidence carries us. 

It further becomes certain, that the whole thickness of the 
floor of the Thames basin, from the Bagshot sands to the 
furthest point reached by the borer, is nothing but mud 
which has been accumulated, by various agencies, at the 
bottom of the sea, and which has subsequently been up- 
heaved. Much of this mud represents the denudation of 
the land surfaces, which were contemporaneous with these 
deposits ; but still more is the work of animal life. Upheaved 
into dry land, the rain has worn and excavated its surface, 
and accumulated into streams, which, gradually cutting 
their way deeper and deeper, have at lenpth, produced the 



XVII.] GEOLOGY OF THE THAMES BASIN. 295 

varied contours of the present Thames basin. Thus, para- 
doxical as it may sound, the river is older than the hills 
and dales amongst which it flows, and which appear to 
determine its course. 

If the question be asked, how long a time has been 
occupied in the formation of the floor of the Thames basin ] 
the only reply which can be given is, that most certainly it 
was of enormous duration, but that there are no means of 
estimating it with accuracy. The whole mass has been 
constructed, as has been seen, of the products of denu- 
dation or of those of vital processes. There is not the 
least reason for supposing that either of these products were, 
on the average, formed more rapidly in those ancient times 
than they are now ; and there is independent evidence, that 
some of these rocks, such as the chalk, were deposited very 
slowly. It may be taken to be certain that the thickness 
of chalk which represents a year's accumulation in the 
Cretaceous ocean, is but a small fraction of an inch. But 
suppose it were an inch; then, as the chalk beneath 
London is 600 feet thick, it follows that this bed alone 
represents 7,200 years. 

In point of feet, however, not only is it almost certain 
that we should be much nearer the truth, in assum.ing that 
the chalk beneath London took ten times as long as this to 
accumulate ; but, it can be proved, that the strata which 
overlie the chalk, in the London basin, represent but a mere 
fraction of those which have been deposited elsewhere, 
since the time at which the chalk was formed. The most 
niggardly computation which lies within the bounds of 
probability, presents us with a sum total of several hundreds 
of thousands of years, for the time which has elapsed since 
the sea, of which the chalk is the bottom-mud, flowed over 
the site of London. 



296 PHYSIOGRAPHY, [chap. 

The study of the fossils contained in the strata of the 
Thames basin is not only of essential importance in proving 
the changes which have occurred in its physical geography, 
but it brings to light other remarkable facts in the history of 
the region. It has been seen that animals, which now hve in 
both colder and warmer climates than those of the Thames 
basin, are found associated together in the drifts ; and that, 
in the older rocks, the remains are such as resemble the 
present inhabitants of warmer climates. But, while the great 
majority of the animals and plants of the drifts are identical 
with, or very similar to, those which now live somewhere or 
other; the inhabitants of the world in former ages become less 
and less like those which now exist, as we go back in time. 
Thus, although in their general character, the animal 
remains of the London clay resemble those of animals now 
living in hot climates, it is only a small percentage which are 
identical with living forms ; while the rest have altogether 
vanished and become extinct. In the chalk, this feature is 
still more marked. Of the many thousand beautifully-pre- 
served kinds of- animal remains which have been obtained 
from that formation, only a very few of the lower forms are 
identical with species now living. Hence, notwithstanding 
the similarity of the chalk to the Globigerina-oozt, the 
remains imbedded in the former at once distinguish it from 
the modern deposit. 

The common-sense reasoning which deduced from the 
facts of the Cannon Street section, the conclusion that a 
people, having the language and customs of the ancient 
Romans, preceded the English inhabitants of that locality, 
applied to the subjacent strata, has permitted no doubt that, 
at some period before the Roman occupation, the Thames 
valley was the haunt of savages armed with flint weapons ; 



XVII.] GEOLOGY OF THE THAMES BASIN. 297 

that elephants and rhinoceroses, bears and hyjenas, roamed 
through its forests ; and that the hippopotamus wallowed in 
the streams of what was, in all probability, a river of much 
larger dimensions than the present Thames. Arguments of 
similar cogency have led to the conclusion, that the solid 
floor of the Thames basm, throughout the thirteen hundred 
feet of thickness which have been directly explored, owes its 
origin to agents of denudation and reconstruction, such as 
are at work at the present day ; and testifies to the general 
uniformity of nature, throughout a period which must be 
counted by hundreds of thousands of years. 

Looking at the record of the past history of the Thames 
basin, as it now lies before us, it would appear to indicate an 
uninterrupted progress from marine to terrestrial conditions 
• — as if the bottom of the ancient sea had been gradually 
upheaved and converted into diy land, after the deposit of 
the Tertiary strata. But, it must be recollected, that the 
ordinary stratified deposits accumulate only under water. 
A dry land surface leaves no indication of its existence, 
except so far as it may support fluviatile, or lacustrine, 
deposits ; or be overgrown by a vegetation, thick and strong 
enough not to be swept away in the next period of sub- 
mergence. Thus it is possible, and indeed probable, that 
the ancient rocks which lie beneath the chalk and gault, were 
upheaved and remained as dry land, for an immense period 
after their formation, and were submerged, and became part 
of the floor of the ocean, only at the end of the Secondary 
period. The chalk may have been a dry land surface for ages 
before the formation of the London clay estuary ; and the 
greater part of the London clay itself, with its superjacent 
Bagshot beds, has probably been dry land, ever since the 
latter were first upheaved. 

There is no reason to believe that any one of this vast 



298 PHYSIOGRAPHY. [cii. xvii. 

succession of changes in tlie physical geography of the 
Thames basin, has been brought about otherwise than 
gradually. In all probability, no human inhabitant of the 
region which has been the seat of these changes, would have 
been more inclined to doubt that things always had been 
as he saw them, that we are at present. And, although it 
is demonstrated that the living population of the region has 
undergone so thorough a change, that hardly a species which 
inhabited sea, or land, in the Cretaceous period, is to be 
found in, or near, the Thames basin of the present time ; it 
is probable that, at any given epoch, the most observant 
and accurate naturalists might have continued their obser- 
vations for centuries together, without being able to discover 
that the forms of animals and plants were other than fixed 
and permanent. Twenty generations of day-flies, however 
sharp their eyes, would fail to make out that the planet 
Uranus changes its place in the heavens. 



CHAPTER XVIII. 

DISTRIBUTION OF LAND AND WATER. 

The disquisitions contained in the preceding chapters have 
been devoted to the description and the elucidation of the 
phenomena offered to common observation by a single 
river-basin — that of the Thames. But, it has been in- 
cidentally remarked, that this is only one of the many river- 
basins of Great Britain ; and it is now needful to see what 
lies beyond its boundaries. If we cross its northern water- 
parting we enter the basin of the Ouse ; if we proceed west- 
wards, we pass /nto that of the Severn ; while, to the south, 
lie the basins of the Medway, which is almost an affluent of 
the Thames, and of several smaller rivers. Each of these 
basins, or any which lie beyond them, might have served 
as our text ; though few are so well adapted for the purpose 
as that of the Thames. 

Passing from river-basin to river-basin, the observer 
would find bolder rehefs than he has met with in the 
Thames valley, in the almost mountainous hills of Wales, 
Cumberland, and Scotland ; the strata would often possess 
a different composition, and would contain organic remains 
of other kind^ and the rainfall and other climatal con- 
ditions would, sometimes, differ widely from those of the 

1 Prof. Ramsay's Physical Geology and Geography of Great Britain 
should be studied by those who desire further information on these subjects. 



300 PHYSIOCxRAPHY. [chap. 

Thames basin ; but there would be nothing new in the 
general truths exemplified by the study of these additional 
facts. 

Whatever direction the journey of the traveller from 
London might take, he would, sooner or later, reach sea- 
water, which, whatever its local name, is really a dependence 
of the Atlantic Ocean ; and he would, thereby, convince 
himself that the land of Great Britain constitutes what 
geographers term an island. Indeed, it is an island of no 
great magnitude ; for there is no point of its surface from 
which the sea coast may not be reached, by three days' good 
walking. 

Of a roughly triangular form, it measures about 600 miles 
from south to north, by about 320 miles from east to west, 
at its broadest part; and its superficial area amounts to 
89,644 miles. In other words, its superficial area is nearly 
equal to that of a square, 300 miles in the side (300 x 300 
= 90,000). There is no subject respecting which people 
have more vague ideas, than in regard to the relative areas 
of the different parts of the earth's surface ; and it will be 
useful, in considering other parts of the world, to take the 
area of Great Britain as a unit of superficial measurement, 
represented by the Roman numeral I. Thus I. will 
represent 90,000 square miles; II. 180,000; \ 45,000, and 
so on. 

Separated from Britain on the west, by a sea which is 
not more than thirteen miles wide, where it is narrowest 
(between Fair-head and the Mull of Cantire) is another 
island of considerable size, that of Ireland. This island 
measures 300 miles from north to south, by 180 miles, from 
east to west, and has an area of 32,513 square miles (-1). 
Moreover, fringing the west coast of Britain, especially in 
its northern part, and extending beyond its northern end, 



XVIIl.j LAND AND WATER. 301 

there are numerous smaller islands, such as the Hebrides, 
the Orkneys, and the Shetlands. There are a few small 
islands off the eastern and the southern coasts ; but only 
one, the Isle of Wight, is of sufficient magnitude to make 
it worth mention. Soundings, taken in the seas which 
surround the group of British islands, show that they all 
rise from a sort of submarine plain, which slopes gradually 
downwards, from the eastward, to the westward. In the 
German Ocean and the British Channel, as the seas in 
the east and south are termed, the depth of water rarely 
exceeds fifty fathoms (300 feet) ; and, to reach a general 
depth of as much as 100 fathoms, we must sail many miles 
to the north of the Shetlands ; or to the west of the 
Hebrides and Ireland ; or still further to the south and 
west of Cornwall, (Fig. 90, p, 302.) ^ 

This shelving plateau is a westward continuation of 
the shores of Norway, Denmark, Holland,- Belgium, and 
France, which form a continuous coast line to the eastward 
of Britain. They are separated from it by an extent of sea, 
which is very narrow (twenty-one miles) at the straits 
of Dover, but gradually widens to the north-east and south- 
west of that point. 

On crossing this narrow strait, the traveller sets foot upon 
the largest continuous mass of land in the world. Starting 
thence towards the east, and bearing a little to the north, he 
might travel for more than 7,000 miles, through northern 
Europe and Siberia, without seeing the sea, until he struck 
upon its shores in Behring's Straits ; at the narrowest part of 
which, he would be separated, by only thirty-six miles, from 
the opposite shore of North America. A more circuitous 
route, through Eastern Russia, and then, by way of Armenia 
and Syria, to Egypt, would enable the pedestrian to travel 
^ From De la Beche's Researches in Theoreiical Geology. 



302 



PHYSIOGRAPHY. 



[chap. 



almost due south, until he again met the sea at the Cape of 
Good Hope, nearly 6,000 miles, in a straight line, from 
his starting-point, China, Burmah, India, Persia, Ai'abia, 
Algeria, Morocco and the Gold Coast, might all be reached 




Fig. 90. — Map showing the effect of an upheaval of the sea-bottom around the 
British Isles, to the extent of loo fathoms. The half-shade represents the area 
which would then become dry land ; the extent of this area is indicated by the 
fact that 1,000 square miles on the same scale is represented by this square I I 

by a foot passenger from any point of this expanse of diy 
land ; the longest dimension of which, from the west coast 
of Africa to Behring's Straits is somewhere about 15,000 
miles. 



XVIII.] LAND AND WATER. 303 

The total area of this great surface of dry land, which, 
with its islands, constitutes the Old World oi geographers, is 
33,682,460 square miles (CCCLXXIV. nearly). Although it 
is surrounded by water on all sides, it no longer receives the 
name of an island, but is termed a co7Ulneiit ; 01 is more 
usually regarded as composed of the three continents of 
Europe, Asia, and Africa. Between the two former, there 
is no natural demarcation, and they would be, for most 
purposes, better spoken of as one region, under the name of 
Eurasia. But Africa is obviously marked off from the rest, 
in consequence of its connection with Eurasia only by a very 
narrow neck of land, the Isthmus of Suez, now cut through 
by the Suez Canal. 

The surface of Eurasia and of Africa is divided into 
river-basins by Avater-partings, and is diversified by elevations 
i.id depressions, like those which have been met with in the 
British Islands, but on a scale proportioned to its relative 
magnitude. It would be beside the purpose of this work 
to study their features in detail ; but the broad aspects of 
the great system, of which the Thames basin forms an 
insignificant part, may be sketched. 

The mountains of England, as we have seen, stand apart 
from its main water-partings ; but those of the Eurasian 
continent coincide with the lines of separation of the great 
water-sheds. A sinuous band of highlands, which often rise a 
mile above the sea-level, and the highest peaks of which some- 
times attain between five and six times that height, stretches 
almost continuously from the waters of the Atlantic in the 
west, to those of the Pacific on the east of the Eurasian 
continent. (See map. Fig. 91.) 

At the western end, this highland zone is narrow and not 
very high, and, as the mountain range of the Pyrenees, 
separates France from Spain; this is followed by the 



304 



PHYSIOGRAPHY. 



[chap. 




XVlii] LAND AND WATER. 305 

broader and higher mass of the Alps, which splits to inclose 
the plain of Hungary ; and is then continued, to the east- 
ward, by the Balkans, the mountains of Asia Minor, and 
those of Armenia and the Caucasus. The highlands of 
Persia and of Beloochistan connect these last with the 
Hindu Kush ; beyond which the elevated land spreads out 
into an immense half-moon-shaped mass, the southern and 
eastern escarpment of which is formed by the Himalayas 
and the ranges continued from them into China; while the 
northern and western escarpment is constituted by the Thian- 
Shian and Altai mountains. The interval between these is 
occupied by lower, but still much elevated, table lands ; 
and the area of these eastern Asiatic highlands is prob- 
ably not less than five-and-twenty times as great as that of 
Britain. 

To the north of this great mountain system, there is an 
enormous plain, which stretches through northern Eurasia 
to the Arctic Ocean. It commences in Europe, in what 
are called the Netherlands, or the Low Countries ; or rather, 
its beginning may be traced in the flat districts of our 
eastern counties, for Britain, as already explained, is but 
an extension of north-western Europe. Thence it is con- 
tinued by the great North-German plain, which stretches 
across Europe to Russia, where its continuity is broken 
by the low range of the Ural mountains ; but only to be 
renewed, on their eastern side, by the vast plains of 
Siberia. To the south of the east-and-west mountain-system 
of Eurasia, there is no similar plain ; and the mountains come 
down much nearer to the sea on the south, than on the 
north, side. In fact, the great line of elevation does not: 
traverse the middle of the continent, so as to divide it into 
two equal halves, one lying on the north and the other on 
the south ; but it runs very much nearer to the southern 

X 



3o6 PHYSIOGRAPHY. [chap. 

than to the northern shores, of the continent. As a con- 
sequence of this arrangement, a section taken across 
Eurasia from south to north would show — first a short slope 
rising rather abruptly from the sea to the top of the 
mountains ; and then, on the other side of the ridge, a very- 
long slope, running gradually down to the sea-level in the 
north. This kind of relief is illustrated, though exaggerated, 
in the contour given in Fig. 92, where a represents a section 
across India culminating in the Himalayas,^/ at d, thesection 
runs across another range called the Kuenlun, which has a 
general direction parallel to the Himalayas. Between these 
two mountain-chains is the elevated plateau or table-land of 
Tibet, r/ at e the Altai mountains are crossed, the land 
between the two ridges d, e, representing the plains of 




abed e S . 

Fig. 92. — Diagrammatic section across Eurasia. 

Mongolia and the desert of Gobi ; and then, from the Altai 
range, the section is continued across the vast Siberian 
plains to the northern sea. 

It is in Eurasia that the highest land in the world is to be 
found. The loftiest known peak is that of Mount Everest, 
in the Himalayas, which rises to a height of 29,000 feet, or 
about 5^^ miles above sea-level. And several other moun- 
tains in the same range attain- to nearly as great altitudes ; 
thus Kanchinjanga reaches the height of 28,178 feet, and 
Doulagiri that of about 27,000 feet. 

It is also in Eurasia that the greatest depressions are to 
be found. The most remarkable of these is that in which 
tlie Caspian sea lies. This inland sea is a body of salt- 



xvin.] LAND AND WATEK. 30? 

water which covers an area as large as Spain, and has the 
level of its surface about eighty-three feet below that of the 
Black Sea ; while the bottom of the hollow in which the 
water rests falls to about 3,000 feet, or nearly three-quarters 
of a mile, below the level of the ocean. The Caspian itself 
occupies the deepest part of an enormous depression, which 
appears to have been connected, at a late geological period, 
with the Mediterranean Sea. Tliis great basin, which also 
includes the inland sea of Aral, covers an area at least as 
large as Central Europe. The Caspian sea alone occupies 
an area of 126,646 square miles {I.^ nearly). The Dead 
sea is another salt-lake much below the sea-level, its surface 
being about 1,300 feet below that of the Mediterranean. 

Since water naturally flows towards the lowest accessible 
level, it is only to be expected that these depressed areas 
should receive the drainage of the surrounding country. A 
large number of rivers do, indeed, discharge themselves into 
these great lakes ; and hence such rivers differ essentially 
from ordinary rivers, like the Thames, in that they never 
reach the open sea. Such streams are often called con- 
tineiital rivers, since they are confined to continental areas, 
and their basins are contained within the land. Thus 
the Dead sea receives the river Jordan ; the Caspian 
receives the Ural and the Volga — the latter, by the way, 
being the longest river in Europe — while the sea of Aral 
receives the Amu Daria (Oxus) and Sir Daria (Jaxartes) 
which come down from the high plateau of Pamir in Central 
Asia. As none of these salt lakes, or inland seas, are in 
communication with the ocean, the water which is brought 
down to them by these rivers must be got rid of by evapora- 
tion ; while the soluble matters, which the rivers dissolve 
from their drainage-areas, must go on accumulating in the 
lake. 

X 2 



3e8 PHYSIOGRAPHY, [chap. 

Africa (11,290,030 square miles, CXXV.), as already re- 
marked, may be regarded as an appendage to Eurasia. In 
historic times, the only connection has been with Asia by 
means of the Isthmus of Suez ; but there are good reasons 
for believing that, even in post-tertiary times, Africa must 
have been also united with Earope ; the connection havinp; 
been effected across what is now the Strait of Gibraltar ; and, 
also, by means of land which stretched over to Italy, and of 
which Malta and Sicily are points still above water. In the 
northern part, Africa has rather an east-and-west extension, 
like that of Eurasia. And, though it has no general axis 
of elevation ; yet, such mountains as it does possess, have 
a tendency to stretch in the same direction. This is seen, 
for example, in the Atlas mountains in north-western Africa, 
and in the Kong mountains parallel with the northern 
shores of the Gulf of Guinea. The southern part of the 
continent extends, on the contrary, in a north-andsouth 
direction ; and the elevated lands of Abyssinia and Zan- 
guebar follow the same course. 

One of the most striking physical features of Africa is 
the great northern plain which forms the desert of Sahara. 
This has an area fifty times as large as that of Britain, 
depressed in some places below the sea-level, but rising, in 
others, to 2,000 feet above it. From the occurrence of 
marine shells in the superficial deposits, and for other reasons, 
it is believed that much of the Sahara is an old sea-bottom, 
which must have been below water at a comparatively recent 
geological period. The proposal to admit the waters of 
the Mediterranean artificially into the depressed portions of 
the desert has been seriously entertained. 

Regions of inland drainage may be found in some of the 
table-lands in the heart of Africa. Lake Tchad, for example 
is a shallow sheet of water which receives the drainace of 



XVIII.] LAND AND WATER. 309 

the surrounding area. This lake has long been known. But, 
within the last twenty years, some very large bodies 
of fresh water have been discovered in the eastern part 
of Central Africa. These include Lakes Tanganyika and 
Nyassa ; the Victoria Nyanza, the Albert Nyanza, and 
the Alexandra Nyanza. The noble sheet of water which is 
called the Victoria Nyanza is about 3,800 feet above sea- 
level, and is probably the largest known body of fresh-water 
at this altitude ; it is believed to have an area of about 
30,000 square miles (|). In this great region of lakes are 
the nead-waiers of two of the most remarkable rivers of 
Africa — the Nile which flows to the north, and the Congo 
which runs to the west. The Nile, which takes its course 
through Abyssinia, Nubia, and Egypt, is especially notable 
for the fact that it runs for more than a thousand miles 
without receiving a single tributary. 

The eastern coast of Eurasia, as we have seen, is washed 
by the Pacific ocean. Lying off its whole length, in some- 
what the same fashion as Iceland and the British islands lie 
off its west coast, and as the Canaries and the Cape de 
Verd islands lie off the west coast of Africa, is a long series 
of outlying isolated masses of land of various sizes, termed 
the Kuriles, the Japanese islands, Formosa, and the Philip- 
pine Islands ; and these are continued, southwards and 
eastwards, by the islands of Celebes and New Guinea. On 
the other hand, the general direction of the southernmost 
prolongation of the eastern end of Eurasia, the Malay 
Peninsula, is continued to the south and to the east, by 
Sumatra and Borneo, and by other smaller islands. They 
rise from an Asiatic submarine plain, just as Britain rises 
from the European submarine plain (Fig. 90, p. 302). 
Borneo has twice the area of Britain, while Sumatra is 
also very large. These Asiatic islands, which constitute the 



3IO PHYSIOGRAPHY. [chap. 

Malay archipelago, are separated, between Bali and Lombok, 
by a narrow, but deep, channel from the Papuan Islands, of 
which New Guinea is the largest. Separated from it only by 
the narrow straits of Torres, is the sub-continental land of 
Australia, which has an area of about 3,000,000 square miles 
(XXXIII.), and is therefore considerably smaller than Europe 
(3,775,400 square miles, XLII.) ; and this, again, is divided 
only by a narrow strait, that of Bass, from Tasmania. 
Nearly parallel with the east coast of Australia, but more than 
a thousand miles distant from it, is a great chain of islands, 
beginning near New Guinea, with New Britain and the 
Solomon islands ; and, with a great break to the south of 
New Caledonia, ending in the islands of New Zealand. 

These islands stand in the same sort of relation to the 
great dry-land area of Australia, as the Japanese and Philip- 
pine islands, of which they are, in a sense, the continuation, 
do to that of Asia. Beyond them, to the eastward, a broad 
zone of the Pacific ocean is dotted over with the small islands 
of Polynesia. 

On looking at a map of the land area which has now been 
described (Fig. 91), it is obvious, that the chief part of it lies 
in the north, and that it tends to thin out into pointed, or 
broken, masses towards the south. The Malay and Papuan 
islands, with Australia on the east, as it were, balance Africa 
on the west ; and if we regard them, for the moment, as 
south-eastern continuations of Eurasia, answering to its south- 
western continuation in Africa ; it will be seen, that the 
eastern coast-line is broadly parallel to the western. In the 
northern parts, the western coast is convex to the west, and 
the eastern concave to the east ; while, in the southern 
parts, the western coast is concave, and the eastern, convex. 
Seventeen hundred miles of sea separate the westernmost 
part of the British Islands from another, considerably smaller, 



XVIII.] LAND AND WATER. 311 

but still great, continental land which stretches for 10,000 
miles from north to south, and has an area of 15,800,000 
square miles (CLXXV.). This is the New World, formed 
of the two almost distinct masses of North and South 
America, joined by the narrow isthmus of Panama. 

It will be observed that the eastern coast of the American 
continent presents the same sort of rough parallehsm with the 
western coast of the old world, as the latter does with its 
eastern coast. Where the one is convex, the other is concave, 
and vice versa ; and the Atlantic ocean lies, like a great wind- 
ing canal, from 800 to nearly 4,000 miles broad, between 
the two. The western coast of the American continent 
would repeat the curvature of the western coast of the old 
world, were it not that, in its northern portion, it trends far 
away to the west, to approach Asia in Behring's Straits- 
Again, in the new world, as in the old, the larger mass of 
land lies to the north, the area of North America standing to 
that of South America, in the proportion of about 17 to 14 ; 
while there is a remarkable similarity of form between South 
America and Africa. But, instead of being much longer, from 
east to west, than it is from north to south, the American 
continent is much longer, from north to south, than it is from 
east to west. 

In accordance with this north-and-south elongation, an 
elevated tract runs from south to north, through nearly the 
whole length of the two continents. Narrow in the south, it 
attains a considerable breadth, and a great elevation in the 
Andes of Bolivia, Peru, and Chile ; in which last country, 
Aconcagua rises to 24,000 feet. After sinking to a mere 
range of hills in the isthmus, it rises and widens out into a 
great table land, which occupies more than a third of tL-a 
width of North America. Several ranges of mountairii, 
known under the general name of the Rocky Mountains, 



317. PHYSIOGRAPHY [chap. 

which have a more or less north-and-south direction, take 
tlieir rise in Mexico and in the western territories of the 
United States, from this table-land, or from its escarpments. 
Just as the east-and-west line of mountains in Eurasia, is 
nearer to the south than to the north coast ; so the north and 
south axis of America is nearer to the west than to the east 
coast. Hence, the western slope of the American conti- 

w E 

a b d 

Fig. 93. — Di.igrammatic section across Norlh America. 

nent is very abrupt, while the eastern side is carried 
gradually down to great plains, which are drained by some 
of the noblest streams in the world ; such as the Mississippi 
in North, and the Amazon, in South, America. If then, a 
section were made across North America, from west to east, 
it would present an appearance something like that repre- 
sented diagrammatically in Fig. 93. Here, it is seen that 




a 

Fig. 94. — Diagrammatic section across South America. 

there is a sharp rise, from the Pacific coast on the west, to 
the Washington range, a; and, thence, to the summit of the 
parallel chain of the Rocky mountains, b. From the 
eastern slope of the Rocky mountains, the section runs 
across the basin of the Mississippi, but rises again before 
reaching the eastern coast. This rise, d, represents the 
Appalachians, which form a range of mountains running 



XVIII.] LAND AND WATER. 313 

parallel to the eastern side of the continent ; and thus 
reproducing, on a smaller scale, the physical features of the 
opposite shore. South America presents a similar section 
(Fig. 94). A very sharp rise leads from the Pacific to the 
range of the Andes, a, whence a broad plain stretches to 
the Atlantic coast, relieved only by the highlands of 
Brazil, b. 

It has been pointed out by Professor Dana ^ that, in all 
parts of the world, the highest mountains border the largest 
oceanic basins. This rule is strikingly illustrated by the 
relief of the American continent. Thus, the Rocky 
mountains, which face the vast Pacific ocean, are con- 
siderably higher than the Appalachians, or AJleghanies, 
which are opposite to the much narrower Atlantic, 

America presents the grandest illustrations of fresh-water 
phenomena to be found in any part of the world. Its river- 
systems are framed on a gigantic scale ; the basin of the 
Amazon, for instance, embracing an area of 1,500,000 
square miles (XVII.), and that of the Mississippi about 
980,000 (XI.) miles. The drainage of the north-western 
part of America is remarkable for its connection with a 
chain of lakes which present a total area of 90,000 square 
miles (I.) of fresh-water. These are known as lakes 
Superior, Michigan, Huron, Erie and Ontario ; and their 
waters are ultimately discharged into the Atlantic Ocean by 
the River St. Lawrence. It is in passing from Lake Erie to 
Lake Ontario, that the stream is suddenly precipitated to a 
depth of 162 feet, to form the falls of Niagara. The 
peculiar chasms through which some of the North American 
rivers run, are illustrated by the Colorado canon figured on 
P- 137- 

^ From whose Manual of Geology the contours of the sections in Figs. 
92 to 94 are taken. 



3'4 



PHYSIOGRAPHY 



[chap. 



The preceding sketch of tne disposition of the form and 
size of the dry land takes no account of many considerable 
islands ; and especially leaves out of consideration those 
which, like Greenland, are covered with ice and snow, and 
are rendered almost inaccessible by the accumulation of 
ice in the sea which surrounds them. (See Fig. 95.) 




Fig. 95. — Map of Arctic regions. 



The total area of dry land has been estimated at about 
52,500,000 square miles (DLXXXIIL). Whether the 
voyager travels south, or north, from the coasts of this dry 
land, his progress is, sooner or later, stopped by the ice, which 



XVIII.] 



LAND AND WATER. 



315 



accumulates in the seas of the cold northern and southern 
regions ; but, without taking the frozen seas into account, 
the area of the ocean is twice as great as that of the land. 
Moreover, though it is doubtful if the sea anywhere attains 
a depth greater than the height of the highest mountains, 
the average depth of the sea is greater than the average 
height of the land above the sea ; so that, in all ways, there 
is much more sea than dry land. 

It has been calculated that of the entire surface of the 
earth, 144,500,000 square miles are covered by water; and 




Fig. 96. — Continental or land 
hemisphere. 



Fig. 97. — Oceanic or water 
hemisphere. 



as there are 52,500,000 square miles of dry land, the quantity 
of water exceeds the quantity of land, nearly in the pro- 
portion that 8 exceeds 3. In other words, for every square 
mile on the earth's surface, there are nearly three square 
miles of water. 

Again, it may be observed that the land and water are 
not uniformly distributed, so as to preserve the same pro- 
portions in all parts of the world. The northern parts 
evidently contain much more land than water, while the 



3i6 PHYSIOGRAPHY. [CH. xvm. 

southern parts contain much more water than land. There 
is, in fact, about three times as much land in the northern 
half of the world as in the southern half. Fig. 96 repre- 
sents that half of the world which contains the greatest 
amount of land, and Fig. 97 the other half, which 
contains the greatest proportion of water. 



CHAPTER XIX. 

FIGURE OF THE EARTH. CONSTRUCTION OF MAPS. 

In considering the form, size, and other characters of the 
Thames basin, we found no occasion to trouble ourselves 
about the shape and size of the earth as a whole ; and, as 
what is true, in this respect, of the area of the Thames basin, 
is true of all areas of the earth's surface, it is obvious that 
all the facts stated in the last chapter might have been 
ascertained by the ordinary processes of land-surveying, 
and that they do not necessarily presuppose a knowledge 
of the configuration of the world. 

One's earliest and most natural impression is that the sur- 
faces of the land and sea are everywhere flat, if local elevations 
are left out of consideration ; and, for many ages, it was tne 
accepted belief of mankind that the land was a huge flat 
cake surrounded on all sides by an illimitable ocean. But 
when, in 1520, Magellan, sailing westward from Europe, 
passed round the southern end of South America ; and, his 
ships keeping their bows continually in the same direction, 
eventually reached the coasts of Asia, and thence returned 
to the place from whence they set out, it was demonstrated 
that, at any race along the track he followed, the surface of 
the earth was round. 



3j8 physiography. [cfivp. 

But without having recourse to a voyage of circumnaviga- 
tion, very simple reasons afford proof that the surface of the 
earth is curved, not only in one direction but in all directions; 
or, in other words, that it has the shape of a ball. 

One of the most commonly cited, but at the same time 
one of the most convincing proofs of this rotundity, is based 






Fig. gS. — Disappearance of a ship at sea. 

upon a simple observation which any one can make for him 
self at the sea-side. If a ship be watched, as she leaves 
port, it will of course be seen that she gets smaller in size 
and fainter in outline the farther she stands out to sea. But, 
in addition to this change of size and of distinctness, the 
figure of the ship suffers a change. In fact, the hull of the 
vessel seems gradually to sink into the sea, and at length dis- 
appears altogether. Yet it might fairly be supposed that the 
hull, being the largest part, would remain longest in view. 



XIX.] FIGURE OF THE EARTH. 319 

After the hull has passed out of sight, the lower sails, in like 
manner, are lost to view ; then, the upper sails appear to dip 
beneath the water ; and, at last, only the tops of the masts 
are to be seen peeping above sea-level (Fig. 98 ^). A telescope 
may make so much as is to be seen of the ship more 
distinct ; but, it will not bring the lower part again into 
view, after it has once been lost. There seems to be no 
way of explaining this gradual disappearance of a vessel 
beneath the surface of the sea, on the supposition that the 
earth is a flat plane ; but the explanation becomes easy 
enough, if it be admitted that the surface is slightly convex. 
Fig. 99 may be taken to represent a section of the sea, show- 
ing the successive positions of a ship as she rides over the 




Fig. 99. — Ship approaching shore, Curvature of sea. • 

curved surface. If the observer is stationed on the tower, on 
the left of the figure, his line of vision might be represented 
by the straight line which runs across the diagram. When 
a ship is at a distant point on the right hand of this figure, 
the observer sees only the top of the masts ; because the 
surface of .the water rises, like a flat dome, which comes in 
the way of his seeing the lower parts of the vessel. But, as 
the ship approaches the shore, the upper sails come into 
view ; then the lower sails are seen ; and, last of all, the 
hull itself. 

To the sailor who is approaching land, similar appearances 

^ Figs. 98 to 100 are t.iken, by Mr. Bentley's permission, from M. 
Guillemin's work entitled T/ie Heavens. 



320 PHYSIOGRAPHY. [chap. 

are presented ; the first points which are visible to him are 
the peaks of hills, or the tops of buildings. He is prevented 
from seeing the bases of these objects by the bulge of 
water, which rises between him and the shore. Now, as 
these appearances are not confined to any one locality, but 
are seen in every part of the world, it follows that the earth 
must have a general curvature. In fact, it can be shown 
that the convexity is everywhere very nearly the same ; and 
it is, therefore, clear that the earth is a globe-shaped body. 

It is possible to obtain similar proof of the earth's round- 
ness by observing a vessel which is stationary. Suppose that 
a person who is about to bathe in a calm sea, sees a small 
boat a mile or two from shore. Let him then get into the 
water, and, with his eye only a few inches above sea-level, 
look along the surface of the water in the direction of the 
boat. He will now find that the boat is more or less 
hidden, or perhaps altogether lost from sight. In fact the 
curved surface of the sea obstructs the view; and the 
obstruction is greater, the lower the position of the bather's 
eye. When a man is standing on shore, his eyes are raised 
something like five feet above ground ; but, when his head 
is in the water, they are only a few inches above the sea 
level, and his view is accordingly obstructed. When the 
observer is in an elevated position, he is able to look over 
the low hill of water which interferes with the prospect at 
lower levels : hence, more of a distant ship can be seen 
from the top of a tower than from its base. 

If a person standing on a wide plain, with nothing to 
obstruct his view, looks round about him, he finds that the 
boundary of his vision extends equally in all directions, and 
thus forms a circle. This boundary is called the horizon} 
The term horizon, at least as used in this sense, therefore 
^ Horizon, from io/fa, horizo, to bound. 



XIX.] 



thp: figure of the earth. 



321 



denotes the circle of vision which seems to separate the 
sky from the earth on land, or the sky from the water on 
sea. But, if the observer mounts a hill, or ascends a 
tower, or climbs to the masthead of a ship, he finds that 
his circle of vision becomes extended, and he can see 
objects which were before beyond his range ; in other 
words, his horizon increases, or becomes a larger circle. 
This is illustrated by Fig. 100. A person standing at the 




FfG. 100. — Enlargement of horizon by ascending a hill. 



foot of the mountain at k^ has his view limited by the 
circle i] if he goes half-way up the mountain to c, his 
horizon expands to the circle marked e; and, if he goes 
quite to the top, s, it enlarges to the circles. If a man's 
eyes are five feet above ground, as they might be if he 
stood at the base of the hill, the radius of his horizon will 
be less than two miles and three-quarters ; but, if he went to 

V 



322 PHYSIOGRAPHY. ("chap. 

the top of St. Paul's, his horizon would then have arailius of 
more than twenty-four miles. 

Since it is found that the horizon is invariably circular in 
e^.'cry part of the world, it is proved that the earth must be 




Fig. ioi. — The eirth within the star -sphere. 

spherical. For a sphere is the only kind of solid which 
presents a circular contour from whatever point it is viewed. 
Other means of demonstrating that the surface of the 
earth is rounded, and not flat, may be obtained from obser- 
vations on some of the heavenly bodies. One interesting 
(Bode of proof may be explamed by the help of Fig. roi. 



XIX.] THE FIGURE OF THE EARTH. 333 

Here the earth is represented as hanging in the centre of a 
great space, which is bounded on all sides by a starry vault. 
Let a person be standing on the earth at the point O. 
That point in the heavens which he sees directly over- 
head, when he looks up, is called the ze7iith; and the 
opposite point, which is immediately beneath his feet, and 
which it is therefore impossible for him to see, since the 
solid earth stands in his way, is called the nadir}- The 
direction of the straight line joining these two points is the 
direction in which a plumb-line hangs when the plummet is 
free. An imaginary plane passing exactly midway between 
the zenith and the nadir constitutes the horizon."^ 

It was explained on p. 9, that very near to the north 
pole of the heavens there is a star called the pole star. 
That point on the horizon which is immediately opposite 
the north celestial pole is the true north, and the other 
cardinal points on the earth's surface are also referred 
to the horizon. Now, suppose a person at O, in Fig. loi, 
observes how high the pole star is above the northern 
horizon ; and that two persons travel from this point — 
one going due north, and the other due south — and that 

^ IVadir and zenith are words of Arabic origin. 

^ There are, in fact, two kinds of horizon. It was said above (p. 320) 
that the horizon is the circle which limits a person'svision, wherever he 
may happen to be. This circular boundary, so far as it is formed by 
the surface of the earth, is distinguished as the apparent, or sensible, 
horizon. The great plane which is shown in Fig. loi passing through 
the centre of the earth, and extended to the celestial sphere, is dis- 
tinguished as the plane of the true, or rational, horizon, which is an 
imaginary circle, dividing that sphere into two equal halves, one above 
and one below the true horizon. Practically these two horizons coincide, 
for the distances of the stars from us are so great that, if the apparent 
horizon were extended until it reached even the nearest of the fixed 
stars, it might be regarded as coincident with the rational horizon, to 
which it would be sensibly parallel, though separated from it by half 
tlie earth's diameter. 



324 PHYSIOGRAPHY. [chap. 

they observe, at different times, the apparent altitude of 
the same star, or its height above their horizon. To the 
man who travels northwards, the pole star will appear to 
mount higher and higher in the heavens ; and, if the ice in 
the arctic regions did not bar him from getting so far, he 
would eventually find this star over his head. In fact, it wall 
be seen from Fig. loi that the pole star is in the zenith of 
an observer at N. But, to the person who travelled south- 
wards, the pole star would appear to be steadily sinking lower 
and lower in the sky ; and, when he got midway between 
the north and south poles of the earth, at the line called the 
equator, he would find that the star actually seemed to touch 
the horizon ; while, if he continued his course to the south, 
it would disappear altogether. But the person who stayed 
at home, at O, would not have observed any movement 
in the position of the star. In fact, it does not sensibly 
change its place ; and its regular movement apparent to the 
travellers has been due to their own change of position on 
the earth's rounded surface, as shown in the figure. This, 
therefore, proves that the earth is convex, as least in a 
north-and-south direction. 

If the travellers, instead of going northwards and south- 
wards, had taken their journey due east and due west, they 
would not have observed any alteration in the altitude of the 
pole star. But, the traveller to the east would have found 
that the sun rose earlier and set earlier than it did when he 
was at O ; while the traveller to the w^est w^ould have found 
that it rose later and set later. It can be shown that this is 
a proof of curvature in an east-and-west direction ; and, there- 
tore, by combining the two sets of observations the rounded 
shape of the earth's surface may be fully established. 

Engineers and surveyors are in the habit of taking the 
earth's sphericity into account in their calculations. If, for 



XIX.] THE FIGURE OF THE EARTH. 325 

example, a canal has to be cut, an allowance must be made 
for curvature in order that the depth of water in the canal 
may be the same throughout. A convincing experiment to 
prove the rotundity of the earth was made by Mr. Wallace, 
in 1870, in the Bedford Level. Three signals, each thirteen 
feet four inches above water-level, were erected at distances 
of three miles apart. On looking through a telescope, 
adjusted, in such a manner, that the line of sight touched 
the tops of the first and last poles, it was found that the 
middle signal was upwards of five feet above the line. 
This rise was of course due to the convexity of the earth's 
surface. 

Such evidence as that which has been adduced in this 
chapter, proves conclusively that the earth has a curved 
surface, and that the curvature is that of a globular 
body. Very delicate operations have enabled men to deter- 
mine the figure of the earth with the greatest accuracy, and 
have shown that this figure is not exactly that of a true 
sphere. The sphere is, in fact, a little flattened in the neigh- 
bourhood of the poles, so that, using a popular comparison, 
it may be likened to the shape of an orange : only it must 
be remembered that the earth's flattening is very much less 
proportionally than that of the orange. In consequence of 
this flattening, a line running round the globe through the 
north and south poles is not exactly a circle, but is an 
elhpse, or something like a circle which has been slightly 
squeezed at opposite points. Fig. 102 is such an ellipse, 
but the extent of flattening is exaggerated prodigiousl3\ 
The polar diameter, or the line which passes through the 
earth's centre, from pole to pole, is found to measure 7 899 '5 
miles There is some reason to believe that the equaioricu 
diameter, or the line which passes through the earth's centre 
from point4o point on the equator, is not the same in all 



326 



PHYSIOGRAPHY. 



[chap. 



directions, and that the equatorial circumference is 
not exactly a circle, but is slightly elliptical; its longer 
diameter measuring about two miles more than its shorter 
diameter. The average equatorial diameter is about 
7 926 '5 miles : in other words the equatorial exceeds the polar 
diameter by about twenty-seven miles. The proportion 
of twenty-seven miles to 7,926 miles is very nearly that of 
I to 294, and hence the earth is said to have an elUpticity 

These variations from the shape of a true sphere are so 
extremely slight, in comparison with the great magnitude of 




Fig. 102. — Diagram showing the kind of difference between the polar and equatorial 
diameters of the earth ; the amount being greatly exaggerated. 

thc^ earth, that, speaking roughly, the earth may for practical 
purposes be called sphere ; and it may be regarded as having 
the shape represented by our ordinary globes. In fact, the 
departure from the spherical shape is too slight to affect a 
model of this kind, unless it is of unusual magnitude.^ 

In order to represent any country by drawing its outline 
upon a globe, or upon a map, it is necessary, in the first 
place, to have some means of fixing the position of places 
upon the surface of the earth. The means by which this 
is accomplished may be easily understood. Suppose that it 

^ In a globe of 2 ft. 6in. diameter, for example, the difierence of the 
polar and equatorial diameters would be very Uttle more than ^^th o-f 
an inch 



XIX.] THE FIGURE OF THE EARTH. 327 

is desired to fix the position of the point P, in Fig. 103 ; let 
any two straight lines be drawn at right angles to each other, 
such as OA and OB, and then measure how far the point 
P is from one of them, say OB. Let P be tliree inches 
from OB, then it is known that the point must be some- 
where in the course of the dotted line CD, which is supposed 
to be three inches from OB. Some clue has thus been 
obtained to the position of the point, but it is not yet 
definitely determined. To fix the point, it is necessary to 
measure its distance also from the other line OA ; let this 
distance be two inches : it is clear that the position of the 
point must be somewhere in the 
line EF, which is everywhere two 
inches from OA. But as it has 
also been shown to be in the line 
CD, it is evident that its position 
is fixed at P, for this is the only 
point at which the two lines cross. 
The distances three and two, re- 
ferred to these lines OB, OA Fig. i03.-Co-ordinatesofapomt. 

respectively, will accurately mark 

the position of P, and they are called by mathematicians 

the co-ordinates of the point. 

Geographers use co-ordinates in this way to indicate the 
position of places upon the surface of the earth. When 
they wish to mark the place of any point, they refer it to 
certain fixed lines which they imagine to be drawn upon 
the surface of the globe. They proceed on the convenient 
fiction that a line is traced entirely round the earth, midway 
between the two poles ; and this line, which is practically 
a circle, they call the equator'^ (Fig. 104). The equator 
consequently divides the world into two equal halves — a 
^ Equator, from Lat. aquo, to make equal. 




3^8 



PHYSIOGRAPHY. 



[chap. 



northern hemisphere and a southern hemisphere. It is 
further supposed that each of these halves is banded round 
by a number of circles, which run parallel to the equator, 
but get smaller and smaller on approaching the poles. These 
circles are called small circles, while the equator is called a 
great circle. The centre of a great circle must be the centre 
of the sphere on which the circle is drawn ; and, it is plain, 
that if the earth were to be cut through at the equator by a 
flat plane, this plane must pass through the earth's centre : 




Fig. io^.— Paralle.s of latiiude. 



Fig. 105. — Lines of longitude. 



but planes passing through any of the small circles, parallel 
to the equator, would not pass through this central pohit. 

The equator serves the place of the line OA in Fig. 103 ; 
it is, in fact, a standard line from which distances may be 
measured. Every circle is divided, for convenience of 
calculation, into 360 equal parts, called degrees; and it is 
supposed that the circumference of the earth is divided in 
this way. The distance of any place from the eqaitor, 
measured along a circle which passes through the poles, and 
expressed in degrees, is called the latitude ^ of that place. 
1 Latitude, from Lat. laiitiido, breadth. 



XIX] THE FIGURE OF THE EARTH. 329 

The distance from the equator to the north pole is one- 
fourth of the earth's circumference, and therefore the latitude 
of the pole is said to be 90°, or one-fourth of 360° measured 
from the equator northwards. In like manner, the south pole 
is in 90° south latitude. London is described as being in 
" 51° 30' N. lat. " ;^ a description which tells us at once that 
London is situated in the northern hemisphere, at a distance 
of 512°, or about 3,560 statute miles from the equator. The 
latitude of so large a place as London will of course vary 
in different parts: the middle of London Bridge is in 51° 
30' 24" N. lat. 

Latitude alone could never fix the position of a place. 
Any number of places, for exampFe, might be situated, 
like London, on the circle which runs round the northern 
hemisphere at 512° ^^ra the equator. Two sets of 
standard lines are therefore needed, just as two lines were 
required in Fig. 103. Geographers have consequently been 
led to draw a number of imaginary circles round the globe, 
all running through the north and south poles, as in Fig. 105. 
These are called lines of longitude, and they differ in several 
respects, beside that of direction, from lines of latitude. All 
lines of longitude form circles which have the earth's centre 
as their centre ; in other woids, they are all great circles. 
But all the lines of latitude, except the equator, are small 
circles. Again, the lines of latitude form equidistant circles, 
and are hence commonly called parallels of latitude. But 
no one can speak of " parallels of longitude," because these 

^ Each deg7-ee of latitude is divided into 60 equal parts called mimUes, 
and eacii minute into 60 equal parts called seconds. Degrees are repre- 
sented by tlie symbol °, minutes by ', and seconds by ". Minutes and 
oeconds of time are distinguislied by tlie initials in and s respectively. 
A minute of latitude is a geographical mile, called by nautical men a 
knot. The geographical mile contains 2028 yards, while our statute 
mile contains only 1760 yards. 



330 PHYSIOGRAPHY. [chap. 

lines are not parallel, inasmuch as they all meet together and 
cross at each of the poles. It is common, however, to refer 
to these imaginary north-and south circles as 7ne7-idia?2s, for 
the reason pointed out on p. 7. 

While latitude is always measured from the equator, 
longitude has no natural starting-line. The reckoning may 
begin indeed from any meridian, and different countries 
actually use different lines for this purpose. The meridian 
from which the reckoning begins is called t\\e Jirst, ox prime 
meridiaji ; and, in this country, it is the meridian which passes 
through the observatory at Greenwich, Greenwich, there- 
fore, has no longitude ; and, in like manner, all places due 
north and south of Greenwich have no longitude, since they 
are on the same meridian. But all places to the east, or to 
the west, of this first meridian have their longitude, which 
is expressed in so many degrees or minutes or seconds, and 
is designated as east or as west, according to its position 
with reference to Greenwich. Thus London Bridge has a 
longitude of 5' 10" W. As the equator is divided into 360 
degrees, it may be supposed that a meridian passes through 
each of the 360 divisions. Hence a degree of longitude 
measured at the equator, is the -j^-oth part of the circum- 
ference of the earth. But in going to the north, or to the 
south, of the equator, the meridians draw closer and closer 
together until they meet at the poles, as shown in Fig. 105. 
Each parallel of latitude, whether large or small, is divided, 
like the equator, into 360°; and, therefore, the length of 
a degree of longitude gets less and less in passing from the 
equator, where it measures 60 geographical miles, to either 
of the poles, where it vanishes altogether. The reckoning of 
longitude proceeds from the first meridian to the east and 
to the west, until the figures reach 1 80° ; the reckoning ot 
latitude proceeds from the equator to the north and to the 



XIX.] THE FIGURE OF THE EARTH. 331 

south, until the figures reach 90°. Hence no place can 
have a greater latitude than 90°, or a greater longitude 
than iSo°. 

It would be too long a story to explain how latitude and 
longitude are practically determined. Unless people happen 
to be mariners, or surveyors, or travellers, they never have 
occasion to fix their position by these means. But, still, 
every one is concerned more or less with latitude and 
longitude, for it is by means of these co-ordinates that we 
can find out any given place upon a map or a globe. The 
cross-lines of latitude and longitude form, indeed, a frame- 
work on which the geographer traces the outlines, which 
show the distribution of land and water upon the surface 
of the earth. 

On a terrestrial globe, it is easy enough to lay down the 
lines of latitude and longitude, and then to draw the outline 
of any country. But when a map, instead of a globe, is to 
be made, it is not so easy to see how these lines should be 
drawn. If the peel were taken off half an orange, it would 
be found impossible to spread this rounded peel upon a flat 
table, without the skin giving way at certain points. A map 
of the world, for this reason, can never give a true repre- 
sentation of the surface of the earth. 

It was said in the first chapter (p. 5) that a map of the 
Thames is an outline-sketch of the river, as it might be drawn 
by a person in a balloon, at a great height, immediately 
above the place which is mapped. This statement is 
perfectly true as far as it goes. As long as the man in the 
balloon looks at the country directly beneath him, he sees 
it in its true aspect ; but, if he looks a long way off, the 
curvature of the earth produces distortion in the distant 
outlines. In one kind of map, however, the person who 
makes it is supposed to be standing at an imraeasa'-abk 



332 



PHYSIOGRAPHY. 



[CHAP. 



distance, and to depict what he sees upon a flat plane 
which is placed between his eye and the earth^ (Fig. io6). 
But his representation will be distorted much in the same 
way as the shadows of objects are distorted when the 
light does not fall square upon their surfaces. Hold a 
plate in the sunshine, in front of a flat surface, and, when 
the light comes down perpendicularly upon it, the shadow^ is 
a true circle ; but, incline the plate, and the circle passes 
into an ellipse ; and, as the plate is inclined more and more, 
so the elliose gets narrower and narrower, until at last, when 





Fig. it>t. — Orthographic projection. 



Fig. 107. — Globular projection. 



the sunlight is passing along the edge of the plate, the 
shadow is reduced to a straight line. The shadow is said 
to be projected on to the flat surface ; and the method of 
throwing a representation of the rounded surface of the 
earth on to a flat sheet of paper is also called projection. 

In the method of projection which has been just ex- 
plained — that in which the eye of the map-maker is supposed 
to be infinitely distant — the central parts of the hemisphere 

^ This is the method of orthograpJiic projection. The parallels of 
latitude become straight lines, as seen in the figure. 



XIX.] THE FIGURE OF THE EARTH, 333 

are accurately represented, but the countries towards the 
circumference are crowded together and diminished in size. 
This defect has led to another method of projection, in 
which the map-maker is supposed to have his eye on the 
very surface of the globe, and to look through the solid 
sphere as though it were a globe of glass, so as to see the 
countries which are on the opposite side ; the outhnes are 
then drawn as they would be projected on a transparent 
screen stretched across the middle of the sphere, and in 
front of the observer's eye.^ 

In this method, it is found that the countries towards the 
centre are contracted, while those near the circumference 
are enlarged. The map is therefore distorted in exactly 
the opposite direction to that of the previous projection. 
Hence it seems natural that if the map-maker took up his 
position at some intermediate point, having his eye neither 
on the surface of the sphere, nor at an unlimited distance 
from it, an accurate representation might be obtained. 
The most favourable point of sight has been calculated, 
and although the view obtained in this way is still distorted, 
the distortion is less than in the other projections. This 
method is consequently the usual v/ay in which the pair of 
hemispheres are drawn (Fig. 107). 

If instead of attempting to represent half the world at 
once, the map-maker is required to represent only one 
country — say Europe — he usually resorts to a different 
device. Imagine a roll of paper screwed up like a sugar- 
bag, and then placed over a model of the globe, like a 
paper extinguisher : this cap will not fit the globe com- 
pletely, but can be made to touch the central parallel of 
latitude of the country which is to be mapped. Let the 

^ This is the method of stereographic projection : the lines of latitude 
fire represented as arcs of circles. Fig. 107 may be taken to represent 
this projection, as it does not differ much from the s^ohular prQJection. 



334 



PHYSIOGRAPHY. 



[chap. 



outline of the country be projected on this cone ; then, 
on unfolding the paper, it may be spread out on a flat 
surface : hence this method is known as that of conical 
development (Fig. io8). Most maps of Europe furnish 
examples of this construction. 

Any of these maps will serve tolerably well for the use of 
landsmen, but they are not what the mariner wants. He 
requires a chart which will give him the true bearing of 
places, so that he may steer direct from one point to 
another, and this he gets by the use of Mcrcator's projection} 




Fig. ioS. — Conical development. 



Suppose the outlines of the various countries of the world, 
and the system of lines of latitude and longitude, were 
depicted on a globular bladder placed inside a glass cylinder ; 
on blowing more air into the bladder it will stretch in all 
directions, and it may be supposed to be sufficiently elastic to 
press on all sides against the inner surface of the cylinder. 
The parallels of latitude then touch the glass, and form circles 
round the cylinder, while the meridians stretch out, and 

^ Mercator was a native of Flanders, born in 15 12. His real name 
was Gerard Kauffman, but, according to the custom of the times, his 
surname, wliich means a ■merchant, was translated into Latin a5 Mercator, 



XIX.] 



THE FIGURE OF THE EARTH. 



335 



form lines which run up and down die length of die cylinder. 
If, when die bladder touches the inside-, it could be ripped 
up, and spread out flat, it would form Mercator's projection. 
(Fig. 109.) All the lines of longitude are straight lines at 
equal distances apart ; and all the lines of latitude are also 
straight lines, but not at equal distances. On a globe, the 
meridians run together near the poles, but on this projection 



I M I I I I I I I I I I I I I I I I i I I I III MM 
~i ~~ 



Fig. 109. — Mercator's projection. 

they are equidistant; hence the high latitudes are evidently 
too much spread out to the east and west, and to counteract 
this distortion, the parallels of latitude are also stretched 
north and south. By thus increasing the distances between 
the parallels of latitude, as they advance from the equator, 
the shape of the land is preserved, but its size is grossly 
exaggerited. The polar regions are not broueht within 



336 PHYSIOGRAPHY. [CH. xix. 

Mercator's projection, for the poles are supposed^ by the 
cyUndrical development, to be indefinitely distant. Such 
a map is therefore not used in the navigation of arctic seas, 
but is otherwise universally employed by mariners. 

For arctic charts the polar projectioti represented in Fig. 
no, is commonly used. Here the parallels of latitude are 




Fig. iio. — Polar projeclion. 

concentric circles around the pole, and the meridians take 
the form of radiating straight lines. The map-maker is 
supposed to have his eye at the centre of the globe, and to 
depict what he sees upon a plane which is at the end of 
the axis, and perpendicular to it. (See the map of the 
Arctic regions, p. 314.) 



CHAPTER KX. 

THE MOVEMENTS OF THE EARTH, 

It has been shown in the preceding pages that the 
waters of the earth are in a state of constant circulation ; 
that the atmosphere is never in repose ; that the solid 
materials of the earth's crust are slowly but incessantly 
changing their position ; and that the matter of the organic 
world is subject, in a yet more marked degree, to cyclical 
changes. Absolute repose is, indeed, a state utterly 
unknown upon the earth's surface. Nor is the globe itself 
exempt from movements which are of a still grander kind. 
The huge ball which was described in the last chapter is 
constantly in motion. Part of this motion is a move- 
ment of rotation, whereby the earth is perpetually spinning 
round like a top ; and part is a movement, of revolution, 
whereby it progresses through space, and is carried round 
the sun. 

If the earth were fixed in space, without either of these 
motions, it is plain that the half which happened to be 
turned towards the sun would enjoy uninterrupted sunshine, 
while the opposite half would be plunged in permanent 
shadow; in other Avords, perpetual day would reign in 
one half, and perpetual night in the other half. The 

z 



333 PHYSIOGRAPHY. |ci!af. 

illuminated hemisphere, on which the sun's rays Avere 
constantly shining, would, of course, become intensely hot ; 
while the darkened hemisphere would become intensely 
cold, by the unchecked radiation of its heat into space. 
Under such circumstances, the hottest part of the 
world would be the middle of the sun-facing hemisphere, 
because the solar rays would there fall square upon its 
surface ; while the heat would diminish, in all directions 
towards the circumference, because the rays would be 
received in a more slanting direction on those parts which 
were farther from the centre of the lit-up half 

If the earth had no atmosphere, the contrast of climate 
between the two halves of the globe would be most intense ; 
for the half turned towards the sun would monopolise all the 
heat sent to it, while the other half would constantly lose heat 
by radiation into space. But, if the earth were enveloped in 
an atmosphere, currents would be raised in this air, and these 
currents would tend to moderate the climate. From the 
highly-heated centre of the illuminated hemisphere, the 
warmed air would rise and spread, on all sides, through the 
higher regions of the atmosphere ; while the less-heated, and 
therefore denser, air from the surrounding parts, would rush 
in, through the lower regions of the atmosphere, to supply 
its place. Hence, any one on the surface of such an earth 
would find winds blowing from all points of the compass 
directly towards the middle of the sun-facing hemisphere. 

If the earth now began to rotate, what would happen 
would depend upon the direction of the imaginary line, or 
axis, round which it turned. The axis coincides with the 
earth's polar diameter, and the points on the surface which 
were described in the last chapter as the earth's poles are 
the extremities of this imaginary line. Suppose, first, that the 
axis coincided with a prolonged radius of the sun, as in the 



XX. J 



THE MOVEMENTS OF THE EARTH. 



339 



first diagram in Fig. iii, where the axis is represented by a 
thick Hne, and the sun, which must be supposed to be at a 
very great distance, is represented by a small circle. Then, 
it is clear, that the same half of the earth would always be 
turned towards the sun ; and the only effect of the twirling 
round would be to modify the direction of the winds, in a 
manner which will be explained presently. But now suppose 






Q 



Fig. iii.«— riagram to illustrate effect of changing the position of the earth's .'ixis 
in relation to the sun. 



that the axis were perpendiadario a prolonged radius of the 
sun, as represented in the second diagram : then the rotation 
of the earth would bring different parts of the surface, in 
succession, towards the sun, and they would thus become, in 
turn, illuminated and warmed. In fact, the rotation would 
produce the alternation of day and night; and the days and 
nights would be equal all over the world, and at all times. 

Z 2 



340 PHYSIOGRAPHY. [chap. 

The poles would be the coldest parts ; and all points of the 
surface, at equal distances from the poles, would be equally 
warmed and equally illuminated ^ while the winds, arising 
from the lower currents, would be directed obliquel)^, from 
the poles towards the equatoi, and those formed by the 
upper currents would blow in the contrary direction. 

Suppose, once more, that the axis were neither in the 
position indicated in the first, nor in that shown in the 
second diagram, but that it occupied some intermediate 
position, such as is represented in the third figure. Here 
it is plain, that the one pole, which is turned towards the 
sun, would always be enjoying a good supply of light and 
heat, while the opposite pole, which is turned away from 
the sun, would be in everlasting darkness and cold. 

As a matter of fact, the axis of our earth is in the position 
represented in the last case ; but in consequence of other 
movements, which will be duly explained, no part of the 
surface is permanently dark and cold. 

If the stars be watched on a clear night, for a short time, 
it will be observed that they appear to move across the 
heavens, from the east to the west, in the same way as the 
sun does during the day ; and, if any one of these stars were 
bright enough to cast a shadow, we might make a star-dial 
for the night, just as sun-dials are made for the day. If the 
star were one of those which never set in England, such, for 
example, as the star in the end of the tail of the Great Bear 
(Fig, i), its shadow would, in the course of the night, sweep 
over a segment of a circle, just as the shadow cast by 
the sun sweeps over a segment of a circle during the day. 
Suppose the circle to be completed, and to be divided 
into 86,164 equal parts; then, observation would show that 
the shad'^w thrown by the star travels over these equal 
parts in equal periods of time, and each such period is 



XX.] THE MOVEMENTS OF THE EARTH. 341 

what is termed a second. Consequently, the shadow would 
return to the same place night after night, in just 86,164 
seconds. If an accurate clock, beating seconds, had a dial, 
the circle of which was divided into 86,164 parts, and a 
single hand, which should move over one of these divisions 
at every beat, the motion of the hand would exactly keep 
pace with that of the star-shadow. And, if any point of the 
dial were marked twelve, when the star-shadow was at 
any point of its course ; whenever the star-shadow returned 
to that place, the hand of the clock would again mark 
twelve. 

Such a clock would keep what is called sidereal^ or star- 
time, and the 86,164 seconds (or 23 hours 56 minutes and 
4 seconds) would be a day by the " star-clock." As the 
apparent motion of the stars is due to the rotation of the 
earth on its axis, the hand of the star-clock vi^ould travel 
round the dial, in exactly the same time as the earth turns 
on its axis; which period of time (86,164 seconds) is 
termed a sidereal day. 

For practical purposes, however, this clock would be of 
very little use. Unless we happen to be astronomers, when 
we ask the time, it is not from any wish to learn how far the 
earth has turned on its axis, in reference to a particular star ; 
we want to know the time of day, or the time of night, 
whether it is forenoon or afternoon, or the like. To 
apply to our sidereal clock for an answer to these questions, 
would be worse than useless. For, supposing that on any 
particular day, twelve by the star-clock exactly corresponded 
with noon by the sun ; the day after, the star-clock would 
mark twelve nearly four minutes earlier ; and, the day after 
that, earlier still by a similar amount ; so that, in a quarter of 
a year, twelve by the star-clock would be six hours before 
noon and so on. In short, twelve by the star-clock 



353 PrIYSIOGRAPHY. [chap. 

might mean any hour in the day or night. The reason of 
this is that day and night depend upon the sun, and the sun 
does not keep sidereal time. In the first place, the interval 
between the time when the shadow on a sun-dial marks noon 
on one day, and the time it marks noon on the next day, is 
always more than 86,164 seconds; and, in the second place, 
the difference is not always the same, but is sometimes more 
and sometimes less. If the sun-dial were a clock, in fact, 
we should say that it did not go very well ; and the only way 
of making a good clock go, in such a manner as to show 
twelve at noon by the sun, or thereabouts, every day, is to 
strike an average of all the irregularities of the sun-dial, 
and add this average to the number of seconds, which 
would be marked by the revolution of the hand of a star- 
clock in the course of a day. 

This average is 236 seconds, which, added to 86,164, 
gives the 86,400 seconds which compose the 24 hours, 
or mean solar day, of civil time. For convenience' sake 
these are counted, not by one revolution, but by two, of the 
hour-hand of an ordinary clock ; and thus the XII. on our 
clocks shows, very nearly, the midday and midnight as 
determined by the sun's crossing the meridian. The coin- 
cidence of twelve by the clock with noon by the sun-dial, how- 
ever, is exact only four times in the year ; at the intervening 
periods, the dial is either faster or slower than the clock. 

Since the earth's figure is nearly spherical, it follows, that 
different points on the earth's surface must move, during the 
daily rotation, with different velocities. Any point on the 
line of the equator will describe a circle equal to the circum- 
ference of the earth. The earth's circumference is about 
24,000 miles ; and, as the rotation is effected in nearly twenty- 
tour hours, the velocity of its equatorial region must be 



XX.] THE MOVEMENTS OF THE EARTH. 343 

something like 1,000 miles an hour. But, on going either 
north or south from the equator, the circle which is described 
by any point on the rotating sphere will be smaller, as is 
shown by the diminution in the diameter of the circles of 
latitude. Yet every point of the surface takes the same 
time to turn once round ; and, therefore, the velocity, or 
rate of motion, must become less and less, as the circlcn 
get smaller and smaller. In fact, at the poles, the velocil/ 
is reduced to nothing. The pole represents simply the end 
of the imaginary line on which the earth turns, and is itself 
stationary. 

Everything on the surface of the earth is necessarily 
carried round with the rotating globe. The atmosphere, as 
shown in Chapter VI. may be regarded as part and parcel 
of the earth ; it forms, in fact, a gaseous shell, which com- 
pletely encases the globe, and shares in all its movements. 
The atmosphere, therefore, moves round at the same rate 
as the surface on which it rests. But this surface rotates, 
as just explained, at different speeds, in different latitudes ; 
and hence the atmosphere, while quiescent over the poles 
moves with increasing rapidity in lower latitudes, until it 
attains 1000 miles an hour at the equator. Therefore, if a 
stream of air starts from one of the poles towards the 
equator, and moves in a direct north-and-south line ; that 
is to say, along a meridian, it will constantly tend to lag 
behind the surface of the earth. At the starting point, the 
air is stationary, because the pole itself has no motion ; 
and, if we could suppose such a stream of air to flow due 
south without coming into contact with anything, the suc- 
cessive points of the earth's surface over which it passed, 
would turn under it with constant' /-increasing swiftness ; 
until, at the equator, they would whirl by at the rate of 
1,000 miles an nour to the east. Imagine the air t!--u£ 



344 PHYSIOGRAPHY. [chap. 

transported from the pole to the equator to come into con- 
tact with the surface of the earth in the latter region. The 
immediate effect upon the bodies at that surface would be the 
same as if they were transported, through still air, at the rate 
of I, GOO miles an hour to the east. That is to say, they 
would seem to be subjected to a frightful hurricane from 
the east ; just as the traveller in a railway carriage, passing 
through perfectly still air, at the rate of sixty miles an hour, 
feels a gale of wind blowing from the dii'ection in which he 
is travelling. 

However, the polar air, in passing south, would soon 
become influenced by the motion of the regions over which 
it passes. It would thereby be deflected towards the east, 
and this deflection would constantly increase, until it reached 
the maximum at the equator. During its passage from high 
to low latitudes, the velocity of the eastward movement im- 
pressed upon the current would have been constantly increas- 
ing. But common experience shows that a body cannot 
accommodate itself, in a moment, to any great change of 
motion. If a carriage suddenly starts, or increases its velocity, 
the passenger is likely to be thrown in an opposite direction 
to that of the movement. The air, in like manner, lags 
behind in passing from high to low latitudes ; and there- 
fore while the earth rotates from west to east, the air, as it 
passes south, acquires a relative motion from east to west. 
Thus the current which started from the north pole would 
acquire, during its course, a relative motion to the west ; 
and, by combining the two motions — that from the north, 
with that from the east — the wind thus produced would seem 
to come from the north-east ; in other words, it would appear 
as a north-easterly, and not as a due north, wind.^ 

1 It may be well to remark that winds receive their names from the 
quarters y>-.'';« which they blow. Ocean currents, however, are generally 



XX.] THE MOVEMENTS OF THE EARTH. 345 

Such a case as this which has just been discussed is by 
no means imaginary. As a matter of fact, a stream of heated 
moist air constantly rises, by its relative lightness, from the 
neighbourhood of the equator, where the heat is greatest 
and the evaporation most rapid. To supply the place of 
the air which is thus raised, colder and denser air rushes 
in from the north and the south of the equatorial belt. Yet 
this inflowing air does not take the shape of a due north 
wind, in the one hemisphere, and of a due south wind, in 
the other hemisphere. The air comes from places where the 
velocity of rotation is less ; and, therefore, it lags behind the 
earth in its rapid rotation from west to east. Hence, the 
wind reaches the equatorial zone from the north-east on 
the north side, and from the south-east on the south side, 
of the equator. In this way it comes about, that winds, 
more or less constant in direction, blow across those parti; 
of the Atlantic and Pacific oceans which lie for some 
distance on the two sides of the equator; the direction 
being from N.E. in the northern tropics, and from S.E. in 
the southern tropics. Such steady winds were of so much 
importance to navigation, before the days of steam-ships, 
that much of the world's commerce depended upon them, 
and they were therefore called trade winds. 

It has just been said that the trade-winds blow in a 
direction " more or less constant." This qualification is 
needed, because the character of the wind is greatly modified 
by local circumstances, such as the distribution of land and 
water, and the altitude of neighbouring land. The trade- 
winds are not equally well marked in the two great oceans ; 
nor are they equally strong at all seasons. 

It may be inquired, what becomes of the air which 

named after the point towards yvhxch. they set. Hence a N.E. wind 
blows from the N.E. ; but a N.E. current flows to the N.E 



34'! PHYSIOGRAPHY. [chap 

rises from the heated equatorial belt ? This air, on reaching 
the higher regions of the atmosphere, flows over the cur- 
rents which are sweeping across the surface below, and 
thus produces currents which drift towards the north in the 
northern, and towards the south in the southern hemisphere. 
But these upper currents are blowing from regions of high 
velocity, to regions of lower velocity, of rotation ; they 
therefore move more rapidly than the earth immediately 
beneath them, and, as it were, outrun the earth in its rotation. 
Hence they are deflected from a simple north-and-south 
course, but in an opposite direction to that of the trades ; so 
that, in the northern hemisphere, they blow from the S.W., 
and, in the southern hemisphere, from the N.W. Such up- 
per currents, moving directly counter to the surface winds, 
may be recognised by their effects on high clouds. In the 
higher regions of the atmosphere, they become chilled ; and, 
at about the thirty-fifth parallel of latitude, they are suffi- 
ciently dense to descend to the surface. Part of this air then 
returns as an undercurrent to the equatorial belt, where it 
becomes heated afresh, and once more rises, thus completing 
the circulation in this part of the atmosphere ; while another 
part of the descending air continues its course as a south- 
west wind, in the northern hemisphere ; and, as a north-west 
wind, in the southern hemisphere ; but these winds are not 
so constant as the trades. The prevalent S.W. winds of 
this country may have, in part, such an origin. These winds 
are the chief rain-bearers to the British Isles ; and hence the 
rotation of the earth is not without its effect upon the water- 
supply of the Thames basin. 

The diurnal rotation of the earth sufficiently explains a 
good many of the apparent movements of the heavenly 
bodies. Thus, every day, the sun appears to rise towards the 



xx.l THE MOVEMExNTS OF THE EARTH vt7 

east, and, after marching across the sky in a curved path, 
to set towards the west. Every night, too, some of the stars 
appear, in like manner, to rise and set ; and this is what they 
must certainly do, if, as we know to be the case, on inde- 
pendent grounds, the earth turns round upon its axis from 
west to east. 

Every one must have noticed in travelling by railway, that 
if his own train is in a station, alongside of another, he con- 
stantly fancies the other train is moving, when it is his own 
which has gently started ; and, on looking out of the window, 
when the train is at speed, it may be really difficult to per- 
suade oneself that the telegraph posts, and the nearer trees 
and houses, are not whirling past the more distant obiects, 
in a direction contrary to that in which the train is moving. 
And although, when one looks at the rising or the setting 
sun, it seems contrary to the evidence of one's senses that 
the sun is not moving and the earth is ; yet this is one of 
the many cases, in which what is called the direct evidence 
of the senses is nothing but a hypothetical interpretation 
of the facts of which sensation tells us. That this appar- 
rently obvious and natural interpretation of the fact of the 
change of place of the sun and stars — an interpretation 
upon which the whole human race were agreed a few cen- 
turies ago — is wrong, and that it is the earth which rotates, 
has long been rendered highly probable; and the experiments 
of M. Foucault some years ago completed the proof. 

The diurnal rotation of the earth does not explain all 
the apparent movements of the heavenly bodies. For 
example, it is observed that the sun does not rise, day after 
day, in exactly the same place. In mid-spring and in mid- 
autumn, it rises almost precisely due east ; but, in midsum- 
mer, it rises to the north of the east point of our horizon, 
and, in mid-winter, it rises to the south of this point. The 



J4S PHYSIOGRAPHY. [chap. 

sun seems, in fact, to change his place in the heavens every 
day, but the circuit of changes is completed in the course 
of a year; so that, next midsummer, he will be again in just 
the same place as that which he occupied last midsummer. His 
apparent movement is, in fact, due to the movement of our 
earth around the sun, in a direction, like that of its rotation, 
from west to east. And, just as the time of one rotation 
of the earth on its axis constitutes a day, so the time of one 
revolution round the sun makes z. year. This revolution is 
completed in about 365^^ days.^ 

It is in consequence of this annual motion of the earth 
that time, told by the stars, differs from time told by the sun. 
It was said above (p. 342), that the sidereal day is nearly 
four minutes shorter than the solar day. The sidereal day 
represents the period of the earth's rotation, but the solar 
day is due, not simply to the rotation; but to this move- 
ment, combined with that of the earth's progressive movement 
through space. The subject is worth examining, because 
it offers one of tlie best proofs of the earth's annual move- 
ment. Suppose that it were possible to see the sun and, at 
the same time, a certain star, on the meridian together, at 
noon to-day ; then, it would be found to-morrow, that the 
star reached the same meridian nearly four minutes before the 
sun came there. But, it is clear, that if the earth simply 
rotated upon its axis, the star and the sun ought to reach 
the meridian at the same time. The delay in the sun's arrival 
is due to his apparent journey in the heavens, which is 
opposite to that of the diurnal rotation of the stars, so that 

^ More precisely 365 days, 6 hours, 9 minutes, 1075 seconds of mean 
solar time. As tlie calendar year contains 365 days, the extra quarter 
of a day gives an additional day every fourth year, or leap year ; and, by 
this addition, the correction necessary to cause the seasons to fall in the 
same months of every year is nearly made. 



XX.] THE MOVEMENTS OF THE EARTH. 349 

the sun appears to move backwards among them. The stars 
are so extremely far off that their apparent position is not 
sensibly affected by our yearly march round the sun ; but the 
sun is so much nearer, that its apparent position is materially 
affected, and the sun, consequently, seems to lag behind every 
day. As a complete revolution is effected in the course of a 
year, -j^-jth part of the journey will be accomplished in a 
day. But a circle is divided into 360 degrees : therefore 
nearly one degree will be travelled over every day. Now the 
360th part of 24 hours is 4 minutes ; hence the change of 
position, due to one day's apparent annual motion of the sun, 
is equal to the change of position due to about four minutes 
apparent daily motion. 

The position which the earth occupies in relation to the 
sun, at different periods of its annual journey, will be under- 
stood by reference to Fig. 112. This shows the earth at 
four successive positions corresponding to the four seasons. 

The track which the sun appears to follow in the heavens, in 
consequence of our motion round him, is called the ecliptic'" ; ^ 
and if it be supposed that a flat surface passes through this 
path, and through the centres of the earth and sun, that 
surface will form thQ plane of the ecliptic, and will coincide 
with the plane of the earth's orbit. 

From what has been already said (p."' 340), it might be 
concluded, that the earth's axis is neither in this plane nor 
perpendicular to it, but that it is inclined to the plane. It 
is in fact sloped, as represented in the figure, at an angle of 
66° 32'; and the extent of the slope remains the same in 
every part of the earth's path ; in other words, the axis may 
be said to remain parallel to itself, and to point to the same 
spot of the heavens. 2 Great as the diameter of the earth's 

^ Ecliptic^ so called because eclipses only happen when the moon is 
either on or very near to this curved path. 

* It should be mentioned, however, that the earth's pole undergoes 



35° 



PHYSIOGRAPHY. 



[chap, 



orbit is, it is insignificant when compared with the enormous 
distances of the so-called fixed stars. If, therefore, the 
north pole of the earth points towards the pole-star at one 
part of the earth's orbit, it continues to point towards it all 
through Its journey, though that journey forms an enormous 
circuit in the heavens.^ 

Reference to Fig. 112 will now show plainly enough how 
the inclination of the earth's axis affects the amount of 





© 





AUTUMNAL 
EQUINOX 

Fig. 112.— Diagram showing the earth's relation 'o the sun at thf.'crent seasons. 



light and heat which the globe receives from the sun at 
different seasons. Suppose that the earth is in the position 
a slow change of position, so that it does not always point to exactly the 
same spot in the heavens. But this movement is so slow that it would 
take 25,868 years for the pole to make'a complete revolution. 

» If one walks in a circle ten yards in diameter, the apparent direc- 
tion of objects fifty yards off will be obviously altered at every step ; 
while the change of bearing of the spire of a distant church can only 
be detected by a theodolite ; and that of a remote mountain top will 
appear to remain unchanged, even with such help. 



XX.] THE MOVEMENTS OF THE EARTH. 351 

which she occupies on June 21, represented at No. i in the 
figure. Here it is seen, that the inchnation of the axis 
causes the north pole to be fully exposed to the sun ; and 
the sun-lit half of the earth includes much more of the 
northern, than of the southern, hemisphere. As the earth turns 
round this sloping axis, the north pole and the surrounding 
parts will continue in sunshine, during the entire rotation. 
Within a circle measuring 23!° from the north pole, the sun 
will not set ; and, within a similar circle around the south pole, 
the sun will not rise. Anywhere, outside the polar regions, 
there will be an alternation of day and night during the 24 
hours ; but the day and night will not be equal, except 
at the equator. Thus, a place in the northern hemisphere, 
like London, will have the day much longer than the 
night ; for the figure shows that it will be kept, during 
rotation, longer in sunshine than in shade. In fact, when 
the earth is in this position, it is inidsiimmer in the northern 
hemisphere ; and, as the figure shows, it is midtmnter in 
the southern hemisphere. These facts are more clearly 
indicated in Fig. 113, which is an enlarged figure of the 
earth in the same position as that of the small globe at 
No. I in Fig. 112. 

As the earth travels round the sun, from June to Septem- 
ber, it completes a quarter of its circuit. The days in the 
northern hemisphere have been getting shorter, and the 
nights longer ; and when the earth has reached the position 
of No. 2 in Fig. 112, or on September 22, it is illuminated 
as represented in Fig. 114.'^ The boundary between the illumi- 
nated and the shaded halves runs directly along a meridian, 

^ From the point ot view from which this figure is supposed to be 
taken, the inclination of the axis is not at first apparent. As the axis 
always points in one direction, it cannot now incline towards the sun as 
it did when in the position indicated in Fig, 113. In both figures, a 



352 PHYSIOGRAPHY. ^chap. 

from pole to pole. Every place on the earth's surface will 
therefore be just as long in sunshine as in shade ; and there 
will be equal day and night throughout the world. 




Fig. 113. —The Earth at the summer solstice. 



In passing from No. 2 to No. 3 in Fig. 112, the nights in 
northern latitudes get longer, and the days shorter. When 
the earth is in the position of No. 3, which is about December 

line drawn from the earth to the sun is supposed to lie in the plane 
of the paper. A plane drawn through both poles coincides with the 
plane of the paper in Fig. 113 ; while, as the earth has moved through 
a quarter of a circle in order to get into the position represented in 
Fig. 114, the plane of the poles is now at right angles to that cf the 
paper ; and the north circumpolar region is seen, foreshortened, in the 
upper part of the figure. 



THE MOVEMENTS OF THE EARTH. 



353 



2 1, it presents exactly opposite conditions of light and 
shade to those represented in Fig. 113. In fact, the north 
pole is now turned as far as possible from the sun, and the 
north-polar regions revolve in darkness, while the southern 
polar regions are enjoying uninterrupted day. 

During the remaining half of its revolution, from No. 3 




Fig. 114 — The Earth at an etjuinox. 



back to No. i, the earth passes through successive stages, 
similar to those just described, but in an inverse order. 
When she has reached the position indicated at No. 4, 
which she will do on March 22, there is again twelve hours 
sunshine in all parts of the world. 

It will now be understood that, at two periods in the 

A A 



354 



PHYSIOGRAPHY. 



[chap. 



couise of the year, when the eartli is at opposite points in 
its orbit, the days and nights are everywhere equal. These 
periods are known as the equinoxes?- One occurs in March, 
and is therefore known as the spring or vernai equinox, and 
the other in September, and is consequently called the 
autumnal equinox. Again, there aie two other periods, also, 
when the earth is at opposite points of its path, when the 
inequality between the days and niglits is greatest. These 
periods are known as the solstices} 

In discussing the earth's revolution round the sun, it 
should be mentioned that the orbit is not strictly a circle, 



PERIHELION 



Fig. 115. — Diagram to i!lustra-e pirihelion and aphelion. The difTereiice between 
the m.ijor and the minor axis is vastly exaggerated. 

but is a curve of that kind which was described in the last 
chapter as an ellipse. In an ellipse (Fig. 115) the longest 
diameter, AB, is called the major axis, and the shortest 
diameter, CD, the w/wr axis. On the major axis, there are 
two points which have this property : — any two lines drawn 
from them to the same point of the curve, are always, 
when joined together, of equal length to the same straight 
line. These two points are known as the foci of the 
ellipse. In the great elliptical path of the earth, the sun 
occupies one of these foci, as at S. It is plain, there- 

^ Equinox, from Lat. teqmis, equal ; nox, night 

' Solstice, from Lat. sol, sun ; sisto, I stand ; because the sun appears 
to stand still in the lieavens at these points in its path. 




xt:.] the movements of the earth. 355 

fore, that, when the earth is at A, it must be nearer to the 
sun than when at B. The nearest distance is called the 
perihelio7i^ and the greatest distance the aphslion? 

It would, at first sight, seem a very fair assumption that 
when the earth is at perihelion it should be hottest, because 
it is then at its nearest point to the sun. As a matter of 
fact, however, the earth is at perihelion about Christmas^ 
which is almost the coldest part of the year, in the northern 
hemisphere ; and it is at aphelion about the beginning of 
July. There are, indeed, several influences which tend to 
neutralize the effects of the sun's proximity. Thus, when 
the earth is at perihelion, the days with us are short, for the 
sun is not long above the horizon. Nor does he rise high 
in the sky, at this season ; and, hence, the rays fall very 
obliquely upon the earth; so that they have less heating 
effect than if they fell more directly upon the surface. 
Again, it must be remembered that the earth moves more 
rapidly as it approaches the sun. These influences more 
than neutralise any augmentation of heat which may be due to 
increased nearness to the source of heat; hence, the apparent 
paradox, that the earth is nearer to the sun during our 
winter than during our summer, becomes easily intelligible. 

It is evident that the temperature of any locality depends 
chiefly on the duration of its supply of sunshine, and on the 
direction in which the sun's rays are received. In this 
country, for example, the temperature is highest when the 
sun has been shining during the long days, and when he 
rises highest in the sky. But the altitude, or height of the 
sun above the horizon, in England, never exceeds about 
two-thirds of the distance from the horizon towards the 
zenith. 

^ Perihelion, from irep}, pert, near ; H\Kios, hclios, the ?un. 
* ApkelioHf from liiro, apo, from ; and ^A.soj. 

A A 2 



356 PHYSIOGRAPHY. [chap. 

At the equator, the sun is directly overhead, or in the 
zenith, at noon in the spring and autumn, and is never more 
than 23!° from the zenith at either solstice ; while the days 
and nights are of practically equal length all the year round. 
Within a circle of 23!° latitude, on each side of the equator, 
there is a belt called the torrid or tropical zone. At every 
place within these zones, the sun is in the zenith twice 
a year, and is never more than 47° from the zenith- 
Hence the intense heat of tropical regions. The boundaries 
of these zones are called t7-opics ; and the countries just out- 
side these circles form sub-tropical regions. 

Around each pole, a circle of 23!° radius m.ay be drawn, 
vhich will include the polar regions or frigid zone. The 
Northern circle is called the Arctic, and the southern is 
called the Antarctic circle. At the poles themselves, the sun 
is above the horizon for six months continuously, and below 
it for an equal period. But, though the polar day is so long, 
the extreme obliquity of the rays prevents the continued 
sunshine from having so great a heating power as it would 
have further south. In fact, at the poles, the sun never 
rises more than 23^° above the horizon. 

Between the polar and the tropical zones, there is, in each 
hemisphere, a broad belt of the earth's surface, which is 
known as the temperate zone. The distribution of the 
surface of the globe into zones is shown in Fig. 116. 

These zones are distinguished, as has just been explained, 
by their differences of climate. The principal factor in the 
formation of climate is, of course, solar heat ; the climate of 
any place depending, primarily, on the lengths of the days 
and nights, and on the relative duration of the seasons. But 
climate is also greatly affected by the nature of the surface, 
whether it be land or water. Water parts with its heat 
much more slowly than the land does, and it thus retains a 



XX.] 



THE MOVEMENTS OF THE EARTH. 



357 



store which serves to equalize the temperature. On land 
again, the climate depends, to a very great extent, on the 
altitude. In fact, on ascending a high mountain, from a 
plain in a hot country, the traveller meets with chariges in 
the character of the animal and vegetable life, which are 
similar to those changes which may be observed in passing 
from low to high latitudes. Even within the tropical 




Fig. ii6. — Zones of the earth's surface. 



zone, the highest points of the existing high lands are 
covered with perpetual snow. Climate is also modified 
by winds, which transport heat and moisture from one place 
to another ; and by marine currents, such as the Gulf Stream, 
the influence of which has already been described (p. 173). 
Climate determines, to a very large extent, the character 



358 PHYSIOGRAPHY. [CH. xx. 

of the animal and vegetable population of a country, or its 
fauna and its flora. In studying the past history of the 
basin of the Thames, as revealed by the organic remains 
described in Chapter XVII., it is evident that this area has 
at different times undergone great vicissitudes of climate ; 
at one time supporting a tropical or sub-tropical vegetation, 
(p. 288), and at another time offering a congenial feeding- 
ground to herds of northern mammals, such as the musk- 
sheep (p. 283). These differences of climate may be par- 
tially accounted for by alterations in the relative distribution 
of the masses of land and water ; but some of the climatic 
changes appear to have been so extreme, that the geologist 
has been led to seek their explanation in astronomical 
causes.^ 

1 See, for example, Mr. Croll's Climate and Titne, 1875. 



CHAPTER XXL 



THE SUN. 



Reference has frequently been made, in the course of the 
foregoing chapters, to the effects of solar heat upon the 
earth. Such references, however, have been incidental 
rather than direct ; and little, or nothing, has been said, of 
set purpose, about the sun itself. It is proposed, therefore, 
in this concluding chapter, to give a simple sketch of 
what we know about the nature of the sun ; and to show 
that the influence of this body may be regarded as the 
prime mover in the production of most of the phenomena 
which are exhibited within the basin of the Thames. 

When the sun is shining in its full splendour, it is much 
too dazzling an object to be looked at with the unprotected 
eye. Viewed, however, through a misty atmosphere, or 
through a dark-coloured glass, it presents the appearance of 
a luminous disc, which is usually perfectly circular in shape 
and homogeneous over its entire surface.^ The size of this 
bright disc does not, however, remain precisely the same 
throughout the year. It was explained in the last chapter 

^ " Usually," because the shape of the disc is sometimes distorted by 
atmospheric refraction, while its uniformity is occasionally, though 
rarely, broken by dark spots large enough to be seen by the unassibled 
eve. 



^6o 



PHYSIOGRAPHY. 



[chap. 



(p. 354) that, in consequence of the shape of the earth's 
orbit, we are not always separated from the sun by the 
same distance; being, in fact, much nearer in December 
than in July. This difterence of distance causes a corre- 
sponding difference of apparent magnitude in the solar disc. 
The apparent size of an object, as every one knows, varies 
according to the distance at which it is viewed, so that a 
halfpenny, held at arm's length, may actually seem larger 
than the entire sun. 

Suppose that an object is situated at A B (Fig. 117) ; its 
apparent height will be measured by the inclinadon of the 
two lines, A O, B O, which are drawn from the opposite ex- 
tremities of the object, to the centre of the eye. A larger 




FiG- 117. — Figure to sliow that apparent niagnitude depends upon the visual angle. 



object will give a larger angle, and a smaller object a smaller 
angle. The apparent magnitude of an object will therefore 
depend upon the angle which it forms, or subtends, at the 
eye. If a small object, C D, be interposed in the line of 
sight, it may be so placed as to subtend prei isely the same 
angle. Hence, a small object, near to the eye, may appear 
just as big as a large object which is a grcdt way off. 

It is easy to see from such a diagram as Fig. 117, how the 
actual size of the sun may be measured. Let a circular 
disc, say one inch in diameter, be cut out of cardboard ; or 
laKe the halfpenny mentioned above, for this has exactly 



XXI.] THE SUN. 361 

the diameter of one inch. Hold the disc, or the coin, at 
such a distance from the eye that it just covers the solar 
disc; keeping the object, of course, square to the line of 
sight. It will be found that the required distance is about 
nine feet. Now on reference to Fig. 117 it will be seen 
that the object A B has exactly twice the height of the 
object C D, and that it is also placed at exactly twice the 
distance ; under these circumstances the two objects appear 
to the eye at O to have precisely the same height. To speak 
generally, the actual heights of two bodies which have 
the same apparent height, are directly proportional to their 
distances. Hence, the distance of the halfpenny bears to 
the distance of the sun just the same proportion that the 
actual diameter of the halfpenny bears to the actual diameter 
of the sun. The actual diameter of the sun is therefore 
found by a simple rule-of-three sum ; ^ provided, of course, 
that the sun's distance be known. Astronomers have 
measured this distance, by methods too complicated to be 
described here, and have found it to be rather more than 
ninety-one million miles.^ Hence, it follows, that the diameter 
of the sun — that is to say, the distance measured from 
side to side through the centre of the sun — is about 852,900 
miles. The sun's diameter is therefore more than 107 times 
as great as the earth's diameter. 

This comparison refers only to diameters. If sections of 
the sun and of the earth were made exactly through the 
centre of each, the area of the sun's section would be 107 

^ It will be understood that this rough method is simply introduced to 
illustrate \h.e principle on which such measurements are made. 

^ As the earth is nearer to the sun at one season than at another (p. 
354), the mean, or average distance, may be taken. The sun's greatest 
distance from the earth is 92,963,000 miles, and its least distance 
89,897,000 miles; hence the mean is 91,430,000 miles, or about 107 
diameters of the sun. 



362 PHYSIOGRAPHY. [chap. 

times 107 times as great as the earth's section. And if the 
'lohimes, or bulks, of the two bodies were compared, it 
would be found that the sun's bulk is 107 X 107 X 107 
times greater than the earth's bulk. In other words, it 
would require more than a million and a quarter of bodies, 
having the same bulk as the earth, to be rolled together, in 
order to form a globe equal in size to the sun. 

No adequate notion of the dimensions and of the distance 
of the sun is gained by casting the eye over the figures, 
which represent these magnitudes. But some conception of 
its enormous size may be formed by reference to Fig. 118, • 
which shows a section of the sun, taken through its centre, 
compared with a similar section of the earth. It was shown in 
Chapter XIX. that the earth is a ball of vast dimensions ; 
but it is, seen by Fig. 118, that this huge ball sinks into 
utter insignificance when compared with the mighty sphere 
around which it revolves. 

As to the distance which separates the sun from the earth, 
that may be represented in a variety of ways ; but, by none 
perhaps more strikingly than by that which Sir John 
Herschel has employed. He tells us that the ball of an 
Armstrong loo-pounder quits the gun with a speed of about 
400 yards per second. Now if this velocity could be kept 
up it would require nearly thirteen years before the ball 
could reach the sun ! 

Soon after the discovery of the telescope, it was directed 
to the examination of the solar disc. ' It was thus found, in 
the beginning of the seventeenth century, that the sun's 
face, instead of being uniformly bright, is usually spotted 
with patches which appear dark, inasmuch as they are less 
luminous than the intensely bright surface which surrounds 
them. Very little observation is needed to show that these 
spots are not constant, either in shape or in position ; 



xxi.l THE SUN. 363 

sometimes indeed, though but rarely, they are altogethei 

absent, and the face of the sun then seems perfectly pure. 
If the spots are watched day after day, they may be seen to 

march slowly across the disc, all moving in the same 

@ 

EARTH 




Fig. 118. — Comparative sizes of the sun and the earth. In the course of engrav- 
ing the circle representing the earth has become a little too large. In order 
to represent the true distance of the earth from the sun, the two figures ought to 
be about twenty -seven feet apart. 

direction from the eastern edge or limb, towards the western 
side, and completing the march in about fourteen days. A 
fortnight afterwards, some of the very same spots v/hich 
were lost may possibly reappear on the eastern edge, though 



364 PHYSIOGRAPHY. [chap. 

altered in shape. This regular movement of the spots 
teaches us that the sun rotates on its axis, and thus 
resembles the earth. One rotation of the sun is accom- 
plished in about twenty-six of our days.^ 

From the different forms which the same spot appears to 
assume, in passing across the disc, it may be inferred that 
the shape of the sun is spherical ; an inference which has 
been abundantly corroborated by other observations. A 
given spot, when near to the margin of the disc, is fore- 













rP'^ 






.^«v-..-«-^ 


.- 3' 


-5^f^-^ ^ 


3%' -■ 








'^% 
^ 






^ 




'?: '' 


•'T' 




,'■^''1 


'■■ s 






_..„r'' U,.^ 



Fig. iig.— The great sun-spoi of 1S65 as it appeared on Oct. 14. (Howlett). 

shortened, and presents quite a different appearance from 
that which it shows when in full view near the centre of 
the disc. Fig. 119 is a central view of a great sun-spot 
which appeared in 1865. 

If the spots always travelled across the disc in straight 
lines parallel to the sun's equator, it would obviously indicate 
that the sun revolved in an upright position ; that is, with 

1 This time differs from the time given above as that at which a spot 
appears and reappears at the same edge of the disc (about 28 days). The 
difference is due to the earth's revohition. 



XXI.] THE SUN 365 

its axis perpendicular to the plane of the earth's orbit. But, 
as a matter of fact, the spots travel in this direction only at 
certain seasons, and, at other times of the year, they may be 
seen moving in curved lines ; the direction of the curve 
being sometimes tov/ards the north, and sometimes towards 
the south. This change of direction is represented, though 
very much exaggerated, in Fig. 120: where the first figure 
indicates the apparent direction of movement in March, the 
second in June, the third in September, and the fourth in 
December. These varying directions of the paths of the 
spots at different periods are easily explained by supposing 
that the sun's axis is not perpendicular, but is oblique to the 
plane of the ecliptic ; so that, sometimes, the sun's axis is 
inclined towards us, and, at other times, is directed from us. 
The rotation of the sun therefore takes place, like the rota- 
tion of the earth, around an inchned axis. The inclination 
of the sun's axis, however, is very much less than that of 
the earth's axis ; being, in fact, only about 7 1° from the 
vertical. 

Observations on the motion of the sun-spots have also 
established the fact that the sun is not strictly a fixed body, 
around which the earth revolves, but that it has a motion of 
its own through space. The earth, indeed, not only revolves 

in a nearly circular path round the sun, but, as it travels, it is 
carried along with the sun at an enormous speed. Hence, 
the actual path which the earth describes in the heavens 
must be compounded of these two motions, and will 
probably be a spiral. 

As so much of our knowledge of the sun has' been 
derived from a study of its spots, it is worth while to inquire 
a little more closely into their nature. Reference to Fig. 119 
will show that a spot is not equally dark throughout ; the 
fringed margin, represented by a half-shade in the figure, is 



366 PHYSIOGRAPHY. [chap. 

called \.ht penni/tbra,^ while ihe darker shade represents the 
uvibra ; and, within the umbra itself, there may sometimes 
be detected a yet darker part, which is called the nucleus. 
There is reason to believe that these spots are nothing but 
gigantic cavities, and that differences of shade correspond 
to differences of depth, the nucleus thus representing the 
most profound part of the hollow. The intensely luminous 
part of the sun, which is the seat of these spots, is called the 
photosphere!^ It appears to consist of incandescent cloud- 
like matter, which is subject to violent disturbances, whereby 
depressions are produced, into which the solar atmosphere 
Rishes from higher regions. The rapid changes in the shape 




tio. 120. — Appiueiu paths of sun-spots at diflerent times of the year. 

of certain sun-spots indicates the violence of this action. 
Some of the spots are so large as to occupy millions of 
square miles on the sun's surface. 

Viewed through a powerful telescope, the whole surface 
of the sun seems to be coarsely mottled. This mottling 
is probably due to irregularities in the cloudy surface. 
Lower levels are indicated by the darkened spots, and, 
in these parts, light is lost by absorption through the 
overlying atmosphere ; where, on the contrary, the light is 
unusually bright, there the sun-clouds are probably unusually 

1 Pemimbra, from foene, almost ; umbra, a shadow. 

» Photosphere, from <f>6os, p/ws, light ; the light-giving sphere. 



XXI. J 



THE SUN. 



367 



high : such brilliant patches generally appear as streaks near 
the margin of the solar disc, and are termed /acu/a:.^ 

Above the luminous photosphere is another envelope, 
which is known as the chromosphere. During a total solar 
eclipse, when the sun is obscured by the moon's shadow, 
the dark disc is seen to be surrounded by a " glory," or 
fringe of radiant light, which is called the corona (Fig. 121). 
Within the corona, around the margin of the disc, variously 




Fig. 121. — The corona and solar prom'nences as seen during a totr.! eclipse in 
iS5[ (Dawes). 



coloured prominences may be detected ; and fantastically- 
shaped tongues of red flame may be seen to dart forth, 
sometim.es to the extent of 70,000 miles or even to greater 
distances. Under ordinary circumstances, these effects are 
not visible, in consequence of the overpowering light of the 
photosphere. But a method has been devised by M. 

' Facula, a little torch, diminutive of Lat. fax. 



368 PHYSIOGRAPHY. [chap. 

Janssen and Mr. Lockyer, whereby these prominences may 
be exammed without waiting for an edipse. Such examina- 
tion has shown that the red flames consist, for the most part, 
of the gas hydrogen (p. 103). Above the region of incan- 
descent hydrogen, there appears to be an enormous enve- 
lope of the same gas in a comparatively cool state. It is 
curious to find that the gas, which forms so large a proporr 
tion of the water of the earth, should be an important 
constituent of the sun. One of the chief chemical com- 
ponents of the river Thames is, in fact, one of the chief 
components of our central luminary. 

It seems almost incredible that a person situated on the 
earth should be able to learn anything about the chemical 
constitution of the sun, which we know to be at least 91 
millions of miles away. To attempt to subject the sun to 
any of the ordinary chemical processes of the laboratory is, 
of course, quite out of the question ; but, within the last 
twenty years a new method of analysis has been introduced, 
whereby a great deal may often be learnt about the chemical 
composition and the physical constitution of an unknown 
body, by proper examination of the light which is emitted 
when the body has been heated until it becomes luminous. 

Without entering into a close description of this method, 
which has been applied with such signal success to the 
examination of the sun, it is sufficient to remark that, when 
a beam of sunshine is allowed to pass through a small 
aperture in the wall of a dark chamber, and then to 
traverse a three-sided glass prism, like the drop of a lustre, 
it does not fall as a spot of white light, but is turned aside 
from its course, and spreads out into a broad band, which 
presents all the colours of the rainbow. This coloured 
band is called a spectru7tt. The course of such a beam of 
light is exhibited in Fig. 122 ; where A is the slit through 



XXI.] 



THE SUN. 



369 



which the light is admitted, and C is the cross-section of the 
prism : instead of faUing as white light at B, the beam is 
deflected from its original path, and widens into a many- 
tinted band, DE, with red at one end and violet at the other 
extremity. 

On closely examining the spectrum of the sun, which 
has thus been formed through a narrow slit, it is found to be 
crossed by a multitude of fine dark lines, which are indeed 
so many spaces in the bright band. A spectrum obtained 
from an ordinary gas-flame, or from the electric light, differs 
from the solar spectrum by being destitute of these dark 




Fig. 122. — Formation of the solar spectrum. 



lines ; the light of the flame being unbroken from end to 
end. But, if certain gases or vapours, such as hydrogen or 
sodium-vapour, be burnt in the path of the artificial light, 
lilies are immediately produced in the spectrum. If the 
temperature of the substance which produces the lines be 
lower than that of the substance which gives the continuous 
spectrum, the lines will appear dark ; if the temperature be 
higher, the lines appear bright. Lines produced in this 
way have a definite position in the spectrum, so that the 
same chemical element, under the same circumstances, always 
gives the same set of lines. It is plain, therefore, that, by 
observing the position of the lines in the solar spectrum, 

B B 



370 PHYSIOGRAPHY. [chap. 

and comparing them with the hnes which are produced by 
the combustion of various terrestrial elements, the presence or 
absence of such elements in the sun may be inferred. For 
the examination of the spectrum, special instruments, called 
spectroscopes, are employed ; and the method of research 
itself, which was suggested and worked out by Profs. Bunsen 
and Kirchhoff, is known as spectrum analysis. 

By means of spectrum analysis, it has been found that the 
sun contains a large number of elements, including hydrogen, 
sodium, lithium, calcium, barium, magnesium, zinc, iron, 
manganese, nickel, cobalt, chromium, titanium, aluminium, 
and copper.^ 

From the surface of the sun, enormous quantities of light 
and heat are continually being radiated, or thrown out into 
space, in all directions. The earth, however, on account 
of its small size and its great distance, can receive but 
an extremely small proportion of the total amount which is 
thus shed. In fact, it is calculated that our globe receives 
less than the two-thousand millionth part of the total quantity 
of the sun's light and heat. All terrestrial phenomena, 
which depend upon solar light and heat, are therefore 
effected by means of this extremely minute fraction of the 
sun's stores of energy. 

The sun is not only the principal source of heat and 
light to our earth, but it is the centre of attraction by which 
the revolving globe is maintained in its regular orbit. If 
a piece of iron be held in front of a powerful magnet, it 
will rush to the magnet, although there is no visible bond 
between them. If the same piece of iron be left unsup- 
ported in the air, it does not remain suspended, but at 
once falls to the ground : in other words, it moves towards 

1 For a detailed account of the constitution of the sun, see Contri- 
hutions to Solar Physics, by J. Norman Lockyer, F.R.S. j 1874. 



XXI.] THE SUN. 371 

the earth, just as it moved towards the magnet ; though in 
neither case is there any visible cause of motion. The 
invisible cause of the motion of the iron towards the 
magnet is called magnetism ; the cause of the motion of 
the iron towards the earth is called gravitatmi?- 

The tendency of bodies to fall towards the earth is called 
their weignt ; and, the nearer bodies on the outside of the 
earth are to the earth's centre, the greater is their weight. 
In consequence of the earth's spheroidal shape, a body at 
the equator is more distant from the earth's centre than 
when at either of the poles. Hence, a given mass of matter 
which causes the index of a spring weighing-machine to 
mark one pound in London will cause it to mark rather 
more than a pound in the polar regions, and rather less 
than a pound in the equatorial zone. If the same body 
could be carried into space, and placed aloof from the 
influence of all gravitation, its weight would entirely dis- 
appear, though the quantity of matter which it contained 
would remain unaltered. 

Gravitation is by no means confined to the earth, but is 
exerted, in greater or less degree, by every mass of matter 
in the universe. Of two bodies containing different quan- 
tities of matter, each tends to move towards the other, but 
the amount of motion of each will be less in proportion as 
its mass is greater. This fact is generally expressed by 
saying that the two bodies attract one another, and that 
the greater the mass, the greater is the intensity of the 
attractive force. Now the sun is a gigantic mass of matter, 
and it attracts all those bodies, including the earth, which 
circulate around it. At present (October, 1877) astronomers 
are acquainted with 182 bodies called //a;/^/^, which revolve 
in regular orbits round the sun. By far the greater number 

1 Grdvifaiion, from Lat. gravitas, weight. 



372 PHYSIOGRAPHY. [chap. 

of these bodies are comparatively small and unimportant ; 
but eight of them are large planets, of which the earth is 
one, though by no means the largest. All these planets 
are retained in their orbits by their gravitation towards the 
sun, which forms the great centre of the solar system. 

Let a ball be tied to a piece of string, and whirled rapidly 
round ; and while revolving in this way, let the string be 
suddenly cut. The ball does not continue to move in a 
circle, but darts off in a straight course, until brought to the 
ground by terrestrial gravity. In like manner, the earth 
would rush into space in a straight path, if the bond of 
gravitation, which plays the part of the string, were broken 
between the sun and the earth. The revolution of the 
earth in a nearly circular orbit is, therefore, maintained by 
means of gravitation. 

Everything upon the earth's surface is subject to terres- 
trial gravity. Every particle of water tends to fall towards 
the centre of the earth, and thus the waters of the ocean 
are bound down so as to form an envelope around the 
globe. But while the water is thus held to the earth, it is 
also attracted by all the other components of the universe; 
and, as the particles of water are free to move, the position 
of any given particle, and, hence, the form of the surface of 
the whole ocean, must be determined, other things being 
alike, by the balance of all these attractions. Most of the 
bodies which lie outside the earth are so distant that their 
influence is inappreciable ; but it is otherwise with the sun 
and the moon. Each of these pulls the water, which lies 
on the face of the globe which is turned towards it, away 
from the solid earth ; while, at the same time, it pulls the 
solid earth away from the water which lies on the opposite 
face of the globe. 

In any parallel of ladtude, which traverses nothing but 



XXI.] THE SUN. 373 

sea, the contour of the latter, if left to the attraction of the 
earth alone, will be, sensibly, a circle. Now suppose the 
sun or the moon to come to any meridian of that parallel, 
then the attraction of these bodies will convert the contour 
of the ocean into an ellipse, of which the long diameter 
will pass through the meridian in question, and that i8o° 
from it ; while the short diameter will traverse meridians 
at 90° from these two. 

If, before the intervention of the sun or moon, the water 
were everywhere of the same depth, it would now be 
deepest at the two meridians, 0° and 180°; and shallowest 
at 90° and 270°. In other words, it would be high water at 
the former, and low water at the latter meridians. 

Supposing the sun or the moon to be stationary, it is 
obvious that, in the course of the diurnal rotation of the 
earth, every point of the ocean under the parallel of latitude 
in question will have been twice raised to the height of high 
water, and twice lowered to the depth of low water ; which 
comes to the same thing as if a wave, with a crest the 
height of high water and a trough the depth of low water, 
had passed twice round the parallel in the same space of time. 

Thus, the rotation of the earth, combined with the attrac- 
tion of the ocean by the sun and moon, gives rise to solar 
and lunar tidal waves. If the free motion of the waters 
of the ocean were not interfered with by the conformation 
of the land, and if there were no moon, high water would 
always take place a little after noon and midnight ; and low 
water would be a little after six o'clock in the morning and 
evening. Moreover, the rise and fall of these solar tides 
would be much less " than our actual tides. For the great 
distance of the sun weakens his tide-producing value to .such 
an extent that his effect, as compared with the moon's, is 
only as 4 to 9, or thereabouts. 



374 PHYSIOGRAPHY. [chap. 

The lunar tides, therefore, are much more important than 
those caused by the sun. If the moon always came to the 
meridian at the same time as the sun (as is the case at new 
moon) it is obvious that the lunar tide would strengthen the 
solar tide, and solar and lunar high waters and low waters 
would correspond. 

Again, if the moon were always 180'^ from the sun (as is 
the case at full moon) the attractions of both would still 
conspire, though not so completely, to the same end, and the 
times of high and low water of both would still coincide. 

On the other hand, if the moon always came to the meri- 
dian six hours sooner or later than the sun, it is obvious 
that the two tidal waves would tend to neutralize one 
another. It would be sun low water when it was moon 
high water, and 7nce versa. In the former cases the high or 
low water would be the sum (or nearly so) of the solar and 
the lunar high or low waters, while, in the latter, it would 
be their difference. 

As a matter of fact, the moon, revolving round the earth 
in a lunar month, comes to the meridian about fifty minutes 
later every day, and constantly changes its position in rela- 
tion to the sun. Hence, in the course of every lunar month, 
there are two periods (new and full moon) when the times of 
solar and lunar high water coincide, and the vertical move- 
ment of the water is greatest ; and two periods (first and 
third quarters) when solar high water coincides with lunar 
low water, and the converse, and when therefore the 
vertical movement of the water is least. The former are 
called spring tides, and the latter ?teap tides?- 

In the open sea, the water is raised by the attraction of the 

It will be understood that only a very general notion of the origin 
of the tides is attempted to be given here. So complicated a subject is 
beyond the scope of this work. 



XXI.] THE SUN. 375 

moon, or of the moon and smi combined, and then falls ; so 
that the true tidal wave represents a mere oscillatory move- 
ment up and down. The lunar wave rises, in the open ocean 
to a height of about 2| feet, and the solar wave to about 
one foot. But, in narrow channels, the tidal wave gives 
rise to a wave of translation, and the water actually 
moves backwards and forwards (p. i8o). This was seen to 
be the case in the tidal part of the Thames, which wasveferred 
to in the opening sentences of the first chapter. 

The movement of the water of the Thames at London 
Bridge, in fact, formed the starting-point of the studies 
which have gradually expanded into these one-and-twenty 
chapters. " What is the source of the Thames ? " was the 
question first proposed for discussion ; but, simple as this 
question seemed, it could not be answered, even in outline, 
until this last chapter had been reached ; and something had 
been said about that vast body, more than ninety millions of 
miles away, around which the earth is constantly circling. 

The Thames is fed, directly or indirectly, by rain ; 
and the rain is condensed from vapour, v/hich has been 
raised into the atmosphere by means of solar heat. With- 
out the sun, therefore, there could neither be rain nor 
rivers ; and hence it is not too much to say that the origin 
of the Thames is ultimately to be traced to the sun. Rain 
is dependent for its distribution upon currents in the atmo- 
sphere, but these currents are due to disturbances of equili- 
brium which are brought about by means of solar heat. 
Without the sun, then, there could be no winds. The 
currents of the sea engaged attention in another part of our 
work ; but here again the sun is the prime mover. Whatever 
view be taken of the origin of such currents — whether they 
are due to the immediate action of winds, or to variations 



376 PHYSIOGRAPHY. [chap. 

of temperature in the water, or to the excess of evaporation 
in one place, over that of another — it is clear that the sun 
is the real agent in the formation of ocean-currents. 

In another chapter, attention was directed to the pheno- 
mena which are presented by cold, and especially to the 
formation of glaciers ; here, if anywhere, it might have been 
supposed that the sun certainly had no part to play. Yet 
it must be remembered that the ice of a glacier is water 
which has been distilled by the sun's heat, and that the 
descent of snow in one place connotes the evaporation of 
water in another locality. Without the sun, therefore, there 
could be no glaciers. 

Considerable attention was given, in several chapters, to 
the phenomena of life, so far as they bore upon the subject 
under discussion. But every one knows that heat and light 
are such necessary conditions for the manifestation of life 
that the earth would become lifeless if sunshine were 
withheld. Without the heat which is derived from the sun, 
the temperature of the earth would fall far below the limit 
at which life can be sustained. Green plants decompose 
carbonic acid, and obtain their supply of carbon, only 
under the influence of sunshine ; and it has often been 
remarked that our stores of coal represent so much sun 
shine of the Carboniferous period. Nor is this a mere 
Avild fancy ; for without the sun there would certainly have 
been no coal. 

In studying the geological structure of the Thames basin, 
it was shown that the country had experienced great changes 
of climate at different periods of its history; and such 
changes depend entirely upon our varying relations with the 
sun. In fact, without the sun, the Thames could have had 
no geological history ; for the upper beds, out of which its 
basin is shaped, are almost exclusively made up of fragments 



XXI.] THE SUN. nn 

which have oeen worn from pre-existing land by means of 
running water ; and the flow of water must be connected, 
directly or indirectly, with the action of the sun. 

And thus we reach, at last, the goal of our inquiry. At 
the furthest point to which we have pushed our analysis 
of the causes of the phenomena presented to us, the sun is 
revealed as the grand prime mover in all that circulation 
of matter which goes on, and has gone on for untold ages, 
within the basin of the Thames ; and the spectacle of the 
ebb and flow of the tide, under London Bridge, from which 
we started, proves to be a symbol of the working of forces 
which extend from planet to planet, and from star to star, 
throughout the universe. 



INDEX. 



A.concagua, 311 
Affluents, 4 
Africa, 303, 308 
Air, 87 

Alluvium, 142, 281 
Alps, the, 141, 154 
Amazon, the, 313 
America, 186 — 188, 311, 312 
Ammonia, 85 
Ampney spring, 37 
Analysis, m 
Andes, the, 64, 311 
Anemones, 247 
Animal remains, 225, 2i6 
Anio, falls of the, 122 
Antarctic circle, 356 

sea, 178 
Anthracite, 244 
Aphelion, 354 
Aquafortis, 86 

Arctic regions, 61, 64, 163, 314 
Artesian wells, 33 
Asiatic islands, 310 
Atlantic Ocean, 42, 167, 174 — 178, 263 — 

267, 270,291, 292, 300, 311 
Atlas mountains, 308 
Atmometers, 69 

Atmosphere, 40, 50, 66, 75, 8g, 106 
Atolls, 254, 260 
Australia, 240, 253 
Auvergne, 121, 194, 203 
Avalanches, 154 
Azote, 79 

B 

Bagnigge Wells, 28 

Bagshot sand, 25 — 37, 287 

Barnsley, rainfall at, 49 

Barometers, 91, 92 

Barrier-reefs, 253, 258, 259, 260 

Basins, 15, 20, 214 (jc'^Thames Basin, &c.) 

Bath, hot springs at, 202 



Battersea, 28 

Bedford Level, 325 

Behring's Strait, 178 

Bermuda, 176 

Better-bed coal, 239 

Bilin, siliceous deposit at, 230, 233 

Black Sea, 307 

" Blocs perches," 164 

Boiling-point, 67 

Borax, preparation of, 203 

" Bore," 180 

Borings for wells, 32, 273 

Bovey Tracey coal, 244 

Boxwell spring, 37 

Brahmaputra, the, 145 

Brain-stone coral, 252 

Brent, the, 5 

Brentford, 28, 281 

Brick-earth, 2S0 — 284, 287 

Bristol Channel, 180 

British Channel, 30, 128 

British Isles, 149,164, 183, 185, 203, 211, 299 

"Brown coal," 244 



Cachar, earthquake at, 188 

Cainozoic series, 290 

Caldy, cave in isle of, 123 

Camden Square, rainfall in, 48 

Cannon Street, section in, 275, 296 

Cations, 135, 137 

Carbonic acid, 80 — 85, 224 

Carboniferous period, 377 

Cardinal points, 6 

Caspian Sea, 306 

Catchment-basins, 34, 145 

Caverns, 122, 283 

Cells, 220 

Celts, 286 

Chalk, the, 37, 166, 213, 273, 290 

"'Challenger " expedition, 176, 177, 192, 

232, 261, 269 
Charcoal, 238 
Charles's Wain, g 



380 



INDEX. 



Charts, 5, 92 

Chemistry of water, 100, 115 

of the sun, 368 
Cherwell, the, s 
Chile,, j86, 187 
Chiltern hills, 16, 17 
Chlorine, 109 
Chromosphere, 367 
Churn, the, 5, 35 
Cicatricula, 225 
Cinder cone, 191 
Cirencester, 4 
Clapham Common, 139 
Clerkenwell, 27 
Climate, 357 
Clouds, 41, 42 
Club-mosses, 241 
Clyde, the, 211 
Coal, 234—245, 377 
Coast, wasting of, i6g 
Coccoliths, 267 
Cocospheres, 267 
Cole, the, 5 
Coin, the, 5, 37 
Colne, the, 5, 36 
Colorado, 124, 135, 137, 162, 20: 
Combustion, 80 
Conduction, 177 
Confluence, 4 
Conical development, 334 
Convection, 177 
Coordinates uf a point, 326 
Corals, 246 — 259 
Cornwall, 46, 30c 
Corona, 367 
Cotteswold hills, 17, 73 
Craters^ 189 
Crayford, 281, 286 
Crevasses, 161 
Crystals, 55, 57 
Cup-cOral, 249 
Cumberland, 46, 130 
Cup-coral, 249 
Currents, 173, 177, 179 
" Cyclops " expedition, 261 



D 

Darent, the, 5 

Dead Sea, 307 

Dee, the, 125 

Degrees, 328 

Deltas, 143 

Denudation, 130, 134, 179, 183, 21 

Dew, 39, 51 

Diagrams, 176 

Diatoms, 230, 26S 

Dip of strata, 24 

Distillation, 73 

Distribution of land and water, 29 

Dogs, Isle of, 142 

"Dolphin" expedition, 261 

Doulagiri, 306 



Dover, straits oi, 301 
Downs, the, 16, 17, 290 
Drainage system, 138 
Drift, 165, 278 
Drifts (or currents), 173 
" Dutch rushes," 232 
Dykes, 193 

E 

Earth, the, 317, 325. 337, 346, 350, 3(^3 

Earthquakes, 185, 187 

Ebb tide, 2 

Ebullition, 67 

Ecliptic, 349 

Egypt, 143 

Electrodes, 102 

Elephant, extinct, 282 

Elk, Irish, 2S3 

Elhpticity, 326 

England and Wales, rainfall o£, 46 

Epsom salts, 124 

Equator, 324 

Equinoxes, 353 

Erith, 281, 2S6 

Estuaries, 181, 182 

Etna, 194 

Eton, 28 

Eurasia, 303, 306 

Evaporation, 50, 66, 129 

Evenlode. the, 5 

Everest, Mount, 306 

Evolution, 226 

Ewen springs, 37 



Faults in strata, 30, 215 

Fauna of Thames Basin, 282 

P'erns, 235 

Finchley, 165 

Fixed-air, 82 

Flemingites, 240 

Flint-implements, 2S5 

Floods, 2, 133, 142 

Fluids, 88 

Fogs, 44 

Foraminifera, 266. 270, 291 

Foraminiferal land, 260 

Forests, ancient, 212,242 

Fossils, 225, 229, 288 

France, extinct volcanoes of, 193 

Freezing-point, 17S 

Freshets, 142 

Fringing-reefs, 253,558, 259 

Fronds, 235 

Fruits, fossil, 288 

Fuller's earth, 36 



Ganges, the, 145, 147, 161 
Gases, 86, 245 
Gault, the, 37, 297 



INDEX. 



381 



Geneva, Lake of, 141, 143 

Geological sections, 23 

German Ocean, 301 

Geysers, 124, 201 

Giant's Causeway, 203 

Gibraltar, strait of, 308 

Glacial period, 164 

Glaciers, 155 

Glass, moisture exuded by, 51 

Globigerinae, 265, 268, 291, 292, 296 

Globular projection, 332 

Graham Island, 198 

Grasses, 232 

Gravels, 27, 132, 140, 27S — 280 

Gravitation, 371 

Grays, 281 

Gray's Inn Lane, flint-implement fro 

285 
Great Bear, 9 

Great Britain, water-partings of, 19 
Greensa"d, the, 37, 293 
Grenelle well, 34 
Greenwich, 330 
Grcund-ice, 153 
Gulf Stream, 44, 173, 357 



H 



Hail, 64 

Hammersmith, 28 

Hampshire basin, 214 

Hampstead, 25 — 27, 288 

Harrow, 27, 2S8 

Hartz, the, 244 

Hebrides, the, 301 

Hemispheres, 315 

Herculaneum, 192, 197 

Heme Bay, 170 

Highgate, 27, 2S8 

High-water mark, 180 

Himalayas, 64, 306 

Hoar-frost, 65 

Holland, 145 

Holywell, 28 

Horizon, 320 

" Horse-tails," 232 

Humber, the, 147 

Hydra machine, 262 

Hydrochloric acid gas, log 

Hydrogen, 86, 103, 115, 368 

Hyetographical (or hyetological) maps, 46 

Hygrometers, 52, 70, 71 

Hygroscope, 70 

Hygroscopic substances, 66 



I 

Ice, 55—57. 67, 150, 177 
Icebergs, 44, 57, 163 
Ice-crystals, 158 
Ice-flowers, 63 
Iceland, 124, igz, 202 



Ilford, 2Sr, 284 
Indian Ocean, 254 
Ireland, 19, 301 



K 



Kanchinjanga, height of, 306 
Kennet, the, 5, 37 
Kentish 'i'own, well at, 273 
Khasi hills, rainfall in the, 48 
Kilauea, 193 



Labrador, 44 
Lagoons, 254, 259 
Lakes, 141, 307 — 309, 313 
Land, upheaval of, 186, 259 
oscillation of, 205, 206 
formation of, by animal agencies,. 

246—271 
distribution of, 294 
area of, 314 
Latitude, 328 
Lava, 191, 192, 203 
Lea, the, 5 
Leach, the, 5 
Lechlade, 3, 14 
Lepidodendron, 240 
Lepidostrobi, 240 
Lias, the, 35 
Lignite, 243,245 
Limestones, 118 

Lion, extinct, in Thames valley, 283 
Lisbon, earthquake of, 188 
Little Hampton, 291 
Living matter, 125, 217 
Loadstone, 108 
Loam, 280 
Loddon, the, 5, 37 
London basin, 31, 33, 213, 215 
London Bridge, the river at, i, 14, 125, 

130, 329, 330, 375, 377 
London clay, 25 — 37, 139, 213, 229, 273, 

288, 297 
Longitude, 328 
Lycopodium, 241 



M 

Magnetic declination, 10 
Magnetic oxide of iron, 108 
Magnetism, 371 
Maidenhead 28 
Mammoth, 283 
Man, early existence of, 285 
Maps, 5, 6, II — 13, 46, 317 
Margate, 12S, 166, 181 
Marl, 280 
Matlock, 121 
Mediterranean Sea, 307 



382 



INDEX. 



Medway, the, 298 
Mercator's projection, 334 
Mercury, 76, .92, 98, 104" 
Meridians, 7, 330 
Mesozoic series, 290 
Metals, 76, 106 
" Meter," 70 
Mexico, 193, 312 

Gulf of, 145, 173, 174, 176 
Miles, geographical, 329 
Mines, 200, 244 

Mississippi, the, 145, 147, 243, 313 
Mists, 44 
Mole, the, 5 
Mont Blanc, 192 
Monte Somma, 197, 198 
Moraines, 159 
Mother of coal, 238 
Mountains, 303 
Mourne mountains, 231 
Mud, 130, 142, 233 
Mud-volcanoes, 202 



N 

Nadir, 323 

Nairn, the, 133 

Naples, 188, 206 

Nautilus, 2S9 

Needles, the, 168, i6g 

Newfoundland, 44 

New River, 37 

Newspaper weather-charts, 92 

New Zealand, 202, 235 

Niagara, 313 

Nile, the, 143, 309 

Nitrogen, 79, 105 

Nore, the, 2, 128, 181 

North, the, how to find, 8 

North Pole, 10 

Nucleus, 366 

Nullipores, 255 

O 

Obsidian, 193 

Ocean currents, 179 

Ock, the, 5 

Oolites, 35, 294 

Ooze, 265, 291 

Orbulinae, 268, 271 

Ordnance Survey maps, 11 

Orthographic projection, 332 

Ouse, the, 298 

Oxford, 3 

Oxide of Mercury, 76 

Oxygen, 78, 103, 115 



Pacific Ocean, 178, 254, 260, 267, 309 
Parallels of latitude, 328 
Paris basin, 34 



Pea-plant, 220 

Peat, 233, 234, 24s 

Pebbles, 166 

" Pele's hair," 193 

Penumbra, 366 

Perihelion, 354 

Photosphere, 366 

Pigeons, 220 

Plain of marine denudation, 183 

Planets, 371 

Plans, 5 

Plants, 220, 225 

Po, the, 161 

Polar projection, 336 

Poles of the earth, 356 

Pole-star, 9, 350 

Polypes, 249, 260 

Pompeii, 192, 197 

Post-tertiary formations, 290 

Potassium, 106 

Potholes, 134 

Protein, 220 

Protoplasm, 220 

Psychrometer, 72 

Pumice, 193 



Quarry water, 22 
Quaternary series, 290 
Quicksilver, oxide of, 76 



R 



Radiolaria, 268, 271 

Railways, geological sections along, 23 

Rain, 39, 116, 130 

Rainfall, 39, 45 — 48, 65, 130, 224 

Rain-gauges, 49 

Rainless regions, 4S 

" Rainy day," 48 

Raised beaches, 211 

Reading, 3 

Reculver, 170 

Red oxide of mercury, 77 

Reefs, 252, 258, 259 

Regelation, 158 

Respiration, 80 

Rey, the, 5 

Rhine, the, 145, 147, 161 

Rhinoceros, extinct, 283 

Rhone, the, 141, 161 

River-basins, 15, 29S 

Rivers, right and left banks of, s 

work of, 130 

floods in, 133 
River-valleys, 139 
River-water, 124 
" Roches moutonn^es," 16a 
Rock-crj'stal, 58, 132 
Rocks, 21, 28 
Rocky Mountains, 202, 31 j 



INDEX. 



.383 



Romans, remains of the, 276 
Romney Marsh, 145 
Rosebridge colliery, 200 
Rust, 75 



Sahara, desert of, 308 
S.ilisbury Plain, 290 
" Salses," 202 
Salt lakes, 307 
Salts, 116, 124, 128 
Sand, 132 
Scale of maps, 11 
Scoria, 192 

Sea, the, distillation of fresh water from, 
73 

water of, 127 

evaporation from, 129 

work of, 166 

temperature of, 177 

level of, 180, 211 

soundings in, 301 

curvature of, 319 
Sea-anemones, 247 
Sea-bottom, 217, 260, 291, 302 
Seathwaite, rainfall at, 46 
Secondary series, 290 
Seismology, 188 
Selenite, 120 

Serapis, Temple of, 206, 207 
" Serein," 41 
Seven Springs, 35, 36 
Severn, the, 17, iSo, 298 
Shell deposits, 282 
Sheppey, Isle of, 170, 229, 288 
Shetlands, the, 301 
Ships at sea, appearance of, 319 
Sidereal time, 341 
Sigillariae, 237 
Silica, 132 
Skaptar Jokull, 192 
Sky, the, 43 
Sleet, 63 

Snow, 55, 61, 63, 67, 154 
Snow-crystals, 61 
Snow-line, 64, 163 
Snowdon, 58 
Sodium, 107 
Solfatara, the, 202 
Solstices, 332, 353 
Sounding, 261, 301 
Specific gravity, 85 
Spectrum analysis, 368 — 370 
Sphagnum, 233 
Sporangia, 241 
Spores, 240, 241 
Springs, 21 — 38 

petrifying, 120 

chalybeate, 124 

hot, 202 _ 
Snring-water, 118, 124 
i-tabice, 197 
Staffordshire,' South, 243 



Stalactites, 122 

Stalagmites, 122 

Steam, 39, 86, loj 

Stereographic projection, 332 

Stigmaria;, 236 

Strata, 24, 214 

Sun, the, 349, 350, 359—37? 

Sun-spots, 362 

Switzerland, 59, 163 

Synthesis, in 

Syreford spring, 36 



Teddington, 2 — 4, 15, 73, 128 
Temperature, rise and fall of, 41 

with regard to evaporation, 68 

of the sea, 177 

below surface of earth, 199, 200 
Tertiaries, Lower London, 32, 213, 273 

280 
Tertiary series, 290 
Thame, the, 5, 129 

Thames, the, at London Bridge, &c., i, 
125, 130, 180 

tides of, 2, 375 

sources of, 3, 73, 3.75 

map of, 6, II 

fall of, 15 

valley gravel of, 28, 278 

rainfall of, 46 

as affected by atmosphere, gg 

origin of, 114 

impurities of, 115, 126 

mineial matter in, 126 

saltness of, 128 

alluvial meadows along, 1 42 

no true delta formed by, 147 

solid matter carried to sea by, 148 

ground-ice in, 153 

estuary of , 169, 181, 182, 212, 246 

rocks beneath, 204, 205 

ancient forests and peat in bed of, 212, 
234 

fossils from cliffs along, 229 
Thames Basin, 15, 20 

springs in, 27, 38, 126 

area of, 35 

rainfall of, 46, 65, 73 

gravel in, 132 

work of rain and rivers in, 139 

effect of denudation on, 148 

effect of ice in, 165 

marine waste in, 169 

formerly beneath the sea, 205 

geological structure of, 272, 377 

fauna of, 282 

past history of, 297 
Thames and Severn Canal, 37 
Thames Head, 14, 36, 37, 118, 120 
Thames Valley, 15, 219 
Thanet, Isle of, 145 



384 



INDEX. 



Thermometers, 71, 91, 151, 199 
Tides, 2, 180, 374, 375 
Torricellian vacuum, 90 
Trade winds, 173, 345 
Trafalgar Square fountains, 33 
Travertine, 121 
" Tripoli," 230 
Tropics, the, 130, 356 
Turf, 233 

U 

Upheaval of land, 186, 218, 302 



Valley-gravel, 28 
Vapour, 66, 67, 86 
Vesuvius, 192, 195 — 198, 203 
Victoria Nyanza, 309 
Volcanic dust, 191 

bombs, 193 
Volcanoes, 185 

W 

Wales, 12, 46, 130, 236, 243, 244 
Wandle, the, 139, 280 
Wandsworth Common, 139 
Water, at London Bridge, &c.,2 — 4 

in the atmosphere, 40 

from the chalk, &c. , 37 

crystallization of, 55 

chemical composition of , 100, 106, 115 

decomposition of, by electricity, loi ; 
by sodium, 107 ; by healed iron, 108 

action of heat on, 105 

formation of, 112 

analyses of, 118 

hard and soft, 119, 120. 



Water, salts of, lime in, 120 

organisms in, 125 

impurities of, 126 

mud in suspension in, 130 

carrj'ing power of, 133 

contraction of, 152 

ice in, 177 

freezing-point of, 178 

high and low, 180, 373 

distribution of, 294 

area of, 315 
" Water-dust," 41 
Water-partings, 18, 19 
Watershed, 18 
Water-works, 120 
Waves, 167, 172, 374 
Weather-charts. 92 
Wells, 28, 36, 126 

borings for, 32, 273 

artesian, 33 
Westminster, former water-s'ipply 
Wey, the, 5 
" White coal," 240 
Wight, Isle of, 214 
Wimbledon Common, 139 
Windrush, the, 5 
Winds, 42, 173, 344 
Windsor, 3 
World, map of the, 304 



Yellowstone Park, Colorado, 202 



Zenith, 323 

Zermatt, glacier of, 156 

Zones, 356 



THE END. 



KICHAns CLAY AND SONS, LILIITITD, LC:-:::C:," AND EUNGAY. 



LIBRARY OF CONGRESS 



020 196 947 5 




